Marine Aquarium Faq

Not open for further replies.


Crazy Crab Lady
Staff member
Global Moderator ⚒️
Jul 23, 2004
Reaction score
Northeastern USA
All of these questions are aimed at beginners seeking to start a first marine tank. As a result, many of these answers are basic and general, and expert-only subjects/options will not be addressed.

This FAQ will be added to over time, although it will remain locked as a reference thread. If you have questions that were not answered here or if you have comments/criticisms/etc. about anything written here, please start a new new thread on the subject. If there are any technical problems such as broken links, please PM me.


Are marine tanks harder to keep than freshwater tanks?
It depends on what kind of freshwater tanks you've kept before. Some freshwater animals are difficult to care for, and some freshwater tanks require a lot of maintenance, so people who have experience with advanced freshwater systems may not find marine tanks to be all that difficult. In general though, the average marine tank is considered a more complicated than the average freshwater tank if for no other reason than that there are more water parameters to deal with (the most obvious one being salinity, but there are others too). Also, even beginner-level marine livestock is more fragile than a lot of beginner-level freshwater animals. Although marine captive breeding is an increasing trend, the majority of marine animals in the trade are still wild-caught, making them less hardy than tank-raised or fully captive-bred animals.

How hard/how much work is it to maintain a marine tank?
It depends on the tank size and how complex the setup is. Small tanks require more frequent maintenance than larger ones, although the periodic work involved in maintaining larger tanks can require more physical effort (more water to move for water changes, etc.).

How much does a marine tank of size x cost?
There isn't a single answer to this. The cheapest solution is always to aim for a basic setup and get equipment, tank, live rock, and livestock second-hand (look for tank breakdowns in your area), and the most expensive way is to get everything brand new and go for a full reef with all the bells and whistles. Setup costs for marine aquariums are usually higher than for average freshwater systems of equivalent volume, and they can be quite a lot higher for full reefs. Marine tanks also have additional, recurring expenses due to the need for salt mix or pre-mixed saltwater.

What are nano and pico aquariums?
The terms “nano” and “pico” refer to the size of a marine tank. Tanks are considered nano-size if they are 20-gallons or smaller, sometimes slightly larger. Pico tanks are quite small, usually 5 gallons and under. The terms are to differentiate smaller marine systems within what have traditionally been called “mini reefs.” A mini reef can actually be quite large, but it is still “minature” when compared to a reef in the wild, hence the term. The size limits quoted for nanos and picos are not exact, and often differences in UK vs. US gallons are ignored. The important thing is the increasing difficulty associated with going into smaller tank size ranges. Nano-range tanks are more difficult to keep than larger systems, and picos are even more difficult than nanos.

What size marine tank is best for a beginner or first marine tank?
Larger tanks, although initially more expensive to set up, will always be more forgiving for beginners. The larger water volume allows for more stability in the event of a problem. Tank parameters like salinity and temperature are easier to control in larger systems. Tanks in the rough neighborhood of 50 gallons in size are usually considered a good size for a first marine aquarium. While it is becoming more common to start with smaller tanks as the hobby progresses, smaller tanks have less margin for error - so doing a lot of research in advance is incredibly important when starting with a small tank like a nano, although picos are almost universally considered too difficult and unstable for a first marine system.

What equipment do I need to set up a marine tank?
It depends on the type of system you want to set up. See HERE for lists of equipment for different types of tanks and typical marine maintenance equipment. Maintenance equipment (such as test kits) are required for all marine systems, so make sure to include it in your initial setup budget!

==== ROCK & SAND ====

I live near the coast. Can I get my rock and sand there?
Almost always the answer is NO. In many areas it is quite illegal to collect rock and sand from beaches even if it is legal to collect animals. Additionally, even if you happen to live in an area where rock and sand are unregulated, the fauna found in a tidal environment is not necessarily compatible with more standard marine aquaria (this is particularly the case for colder climates).

What is live rock?
Live rock (LR) is the most important aspect of biological filtration in a modern marine aquarium. The rock is “live” because it has been colonized with algae and bacteria that give you a ready-to-go biological filter. Higher quality live rock also comes with other fauna. Good quality rock is also very porous and therefore light.

How much live rock do I need?
The most commonly quoted minimum amount is 1 pound per US gallon or 0.5kg per UK gallon (or 1kg per 10L). However, this is a very vague and fuzzy rule given the variation in porosity and therefore density of rocks. Some tanks will have as much as double the minimum amount by weight. There is a live rock section on the aquarium calculator here if you want a rough number to start with:

What is base rock?
Base rock usually means dry aragonite, but sometimes the term is also used to mean low-quality live rock that has been colonized with bacteria but not much else. Although dry base rock can be turned into live rock, it is sometimes more dense and can also require extensive cleaning before being safe to add to an aquarium (particularly if it used to be live rock and was just dried out). The primary benefit of base rock is that it is substantially cheaper than most live rock. Dry base rock will eventually become live rock once added to an aquarium containing live rock, but it can take anywhere from a few weeks to a few months for the colonization to take place.

Can I set up a marine aquarium without live rock?
Although technically possible, it is a bad idea for a first marine tank. You will have a very hard time getting sufficient biological filtration without it. It is far easier to set up a marine aquarium with live rock. The only systems where utilizing live rock is a bad idea are hospital tanks where medications are used that would kill off the fauna on live rock.

What kind of sand is appropriate for a marine aquarium?
Aragonite sand is by far the best/easiest to work with, because of its capacity to buffer pH and KH. You can get aragonite substrates with grain sizes ranging from sugar-sized to what might be considered rubble. Coarser aragonite substrates like crushed coral can be cheaper, but it is also more prone to trapping debris. Choose a grain-size according to what animals you want to keep, as some sand-sifting animals require fine-grained sand.

Can I use play sand?
Play sand is a bad choice for a first marine tank. Most play sand is quartz. While it is technically possible to run a marine tank with a quartz substrate, it will not help to keep the pH and KH stable.

How much sand do I need?
It depends on the animals you want to keep. A 1/2” layer is easily sufficient if you don't want to stock sand-sifting animals. If you want sand-sifting animals, particularly sand-sifting gobies and burrow-forming fish, you will likely require a deep sand bed (DSB) to accommodate their behavior – usually about 4” deep.

What is a deep sand bed (DSB)?
DSBs are thick sand beds, with the term usually applying to beds that are 4”+ deep. However, the term DSB is also used to refer to beds that are thick enough to promote the development of anaerobic zones. The minimum depth needed to promote anaerobic activity varies greatly between systems, although 4” is a safe target if additional denitification is a specific goal for the tank. DSBs can greatly increase the filtration capacity of a system, but they often take months to colonize well with bacteria and must be carefully maintained with an appropriate set of animals. DSBs are considered more difficult to maintain effectively than thinner sand beds.

What is live sand?
Live sand is aragonite sand, usually of mixed grain size, that has been colonized with beneficial organisms, similarly to live rock. It is usually sold in plastic bags that have a shelf life, and the bags may or may not be stamped to indicate that. Live sand can give you a filtration boost faster than dry sand would, but you will accomplish the same result in the long run with dry sand using live rock to colonize it.

==== WATER ====

What is “topping off” a marine tank mean?
Marine tanks evaporate water over time, but only H2O, not salts (although salt deposits often form around the edges of the tanks from splashing and water films evaporating). This means that the salt concentration will rise over time if freshwater isn't added to correct the salinity. Tanks therefore need to be topped off with freshwater at regular intervals. Depending on the tank size and whether the tank is covered, topping off may only need to occur once a week or as frequently as every day. Smaller tanks require more frequent topping-off. To determine how often you need to top-off your tank, check the specific gravity regularly.

Where can I get saltwater?
There are two options: buy a dry salt mix and make it yourself in buckets/jugs (mix it with freshwater to produce seawater) , or buy bottles of pre-mixed saltwater from a local store. For small tanks, buying ready-mixed water is feasible and can be reasonably space and cost-effective. For larger tanks, it is usually easier to mix your own salt due to the volume of water that would need to be transported.

I live near the coast. Can I just use sea water?
It's a very bad idea for the following two reasons: pollution and plankton/microbes. Many beaches are heavily polluted due to runoff from farms, roads, boats, and human habitation. Additionally, natural sea water contains a lot of organisms that don't fare well when bottled up. Plankton and other microbes can die off suddenly, sometimes in a only a few hours, and cause nutrient spikes in the water – something would in turn cause nutrient problems in an aquarium. With very few exceptions, it is much wiser to use synthetic salt mixes.

How is a marine salt mix different from any other type of salt?
Sea water contains a lot more than NaCl (your basic table salt). Other elements include iodide, calcium, magnesium, and strontium. A marine salt mix will include all of these things in appropriate ratios to give average* sea water. Salt addatives for freshwater tanks do not attempt to create an appropriate balance of these trace elements and therefore are not suitable for use with a marine system.

* Sea water compositions vary quite a bit from one location to another, and so do salt mix brands. Salt mixes will provide average levels of most elements rather than reproducing a specific location. Some salt mixes intended for reef tanks specifically (vs. FO/FOWLR tanks) are made to specifically be heavy on calcium content.

What is a good brand of salt?
There is no good answer to this question. Everybody has their favorite. One person will hate brand X while another person will use exclusively brand X. Any reasonably popular standard salt mix will be fine for a first marine tank.

What is RO or RODI water?
RO stands for “reverse osmosis,” and DI stands for “deionized.” RO water has many impurities removed, and RODI has more of them removed.

Do I need to buy my own RO/RODI unit?
Not necessarily. You can buy RO or RODI water from some stores. For larger tanks, it is usually more cost-effective to buy your own unit to make the water at home.

==== LIGHTING ====

Do I need marine bulbs?
For corals, yes. Marine bulbs have spectrums that differ from household and freshwater bulbs. Fish won't care what lighting you use, but in addition to not supporting corals, the wrong spectrum can also promote the growth of nuisance species of algae over other more desirable algaes.

What kind of lighting do I need for corals?
Fixtures with 2xT5 bulbs are usually considered the minimum for undemanding corals like soft corals, with more bulbs being needed for more demanding corals. LED fixtures are also an option that is both low-heat and low-wattage, and there are plenty of LED fixtures on the market meant specifically for reefs. Metal halide (MH) fixtures are often used to accommodate the most demanding photosynthetic animals (such as Tridacnid clams) but also have a very high heat output that can be problematic on some tanks.

I have x-many T5 bulbs. What types do I get?
The boring/easy answer is to have 50% “white” bulbs (usually 10,000K, although some use higher K-ratings) and 50% blue/actinic bulbs in an alternating pattern. Many reefers use this combination with success, and it is how most T5 fixtures are set up out-of-the-box when they include bulbs. There are also T5 bulbs available that highlight other colors better, such as reds and oranges (both of which are unlikely to stand out much with standard 50/50 lighting).


Do I need a sump?
Technically no, but they are very useful when possible to accommodate. If you can have a sump, it's a good idea on most tanks larger than a nano (although some even put them on nanos). Sumps help to keep equipment out of the main display and add extra water volume for stability. Some people can't have sumps due to space and weight limitations. If your floor is not on concrete, calculate the total weight of the sump + tank when filled and consult your local building codes to make sure there are no problems. Some floors may readily support an un-sumped tank of a certain size, but sticking almost double the water volume in the same footprint might not be a great idea.

Can I use a canister or hang-on filter instead of a sump?
Canister filters are not substitutes for sumps. If your floor will support the added weight of a sump, a sump is always the better option for the added water volume (more volume = more stable) and space to put equipment like protein skimmers and chemical media like carbon and phosphate remover. If you can't or really don't want to use a sump, canister and hang-on filters can be used as places to put chemical media. However, canisters and hang-on filters are also prone to accumulating debris and uneaten food, so they require weekly maintenance and are also best used with a prefilter (a sponge or floss cover over the intake), which must also be cleaned/changed regularly.

Do I need a protein skimmer?
Protein skimmers are very useful on most systems, particularly larger tanks. Many people run nano and smaller tanks skimmer-less, since it is easier to do larger and/or more frequent water changes to remove nutrients. Some run larger tanks skimmer-less as well, although skimmers are the norm for most larger-than-nano systems. Protein skimmers remove fine particulates and large molecules from the water before they can decay, thereby increasing the stocking capacity of the tank. Usually protein skimmers must be employed to get nitrates under 5ppm for systems that need to be low-nutrient. Skimmers can also make tanks “too clean” for some filter-feeding and nutrient-hungry organisms, although such animals are also not recommended for beginners (primarily non-photosynthetic corals and Bivalves).

How many power heads do I need / how much flow do I need for a tank of size x?
There is no fixed answer for this, as it depends heavily on the tank's dimensions and how you stack live rock within that. One of the main purposes of power heads is to provide flow around the live rock. Estimates on how much flow is needed can vary quite a lot, but 20x turnover (where x is the tank volume) for the total amount of flow produced in the tank is a commonly quoted starting amount. How much flow a pump contributes is measured in volume of water moved per unit time: gallons per hour (GPH) or liters per hour (LPH). Tanks with <20x flow are fairly uncommon and usually for specialized habitats, but tanks with >20x turnover are quite common (some large reef tanks even go as high as 40x). Ideally you do not want all of the flow coming from a single outlet, since in all but very small systems you are likely to still have "dead spots" in flow that are prone to accumulating detritus, growing algae, and so on. Such setups also create very linear flow that can be undesirable in a reef setting. A few, lower-flow power heads spaced out around the tank us generally better than one incredibly strong power head.

==== LIVESTOCK ====

How should new animals be acclimated to a marine tank?
Many marine animals need some degree of acclimation to a marine tank to survive the differences in chemistry between the bag water and tank water, as water at a pet store is almost never going to be identical to water in your tank. Sometimes the differences can be large enough to kill sensitive invertebrates when the shift occurs too quickly. Fish can handle changes in parameters like salinity much more easily than invertebrates, and so slow methods of acclimation is primarily an issue for invertebrates. Some invertebrates are much more fragile than others. More information on the topic can be found HERE.

What is a CUC (Clean up Crew)?
The term "CUC" refers to an assortment of animals that help to consume uneaten food, eat algae, and maintain the sand bed and live rock of a tank. Often the first CUC animals into a tank are quite hardy: some collection of hermit crabs and grazing snails. Other animals sometimes considered CUC members such as brittle stars and sand-dwelling conches require a more established system. NOTE: sometimes people also refer to a sea cucumber as a "cuc" for short, but these animals should not be considered members of a clean-up crew and do not belong in most tanks.

How many CUC animals do I need for a tank of size x?
This is a difficult question to answer, as every tank is different and the CUC needs can vary over time. Note that the CUC animals that will be supported when a tank is 6 or 12 months old will be very different from what it will support when it has just finished its cycle. Many tanks, even seemingly large ones, may only support a handful of animals when brand new without risking starvation/attrition. Additionally, some CUC animals should only be added once a tank has matured a bit.
Last edited:
Not open for further replies.

Most reactions