Tap vs RO/DI
Tap water can contain phosphates and metals as well as having varying hardness. In a marine tank we control the environment with the salts. The salts have all sorts of trace elements that are particular for a marine tank. I'm not saying it can't be done with tap but if you want to have a more pleasant time of it do the RO thing. I say this from personal experience having started my first marine tank with tap.
Live rock
What the bear minimum of live rock is depends on what you have in the tank and the tank size. In a 75 gallon tank you could get away with 50 pounds of quality live rock if you had only a few fish in there. The rock is your biological filtration as so must have enough surface area on and in it to handle the nutrients. Live rock takes the nitrogen cycle a step more than fresh water does. Deep in the rock anoxic bacteria grow that convert the nitrate into nitrogen gas which then leaves the tank. It's possible to have 0ppm nitrates in a reef tank due to this action. You can always add more rock later if you need to but for aquascasping purposes I like to get it all at once.
Below is some excerpts from my build thread on the reef forum where I hang out. Sorry for this being so long a post but it should give you an idea of how the rock can be stacked and managed in the tank. Side note, always put the rock in first then the sand. That way when you have burrowing creatures they don't undermine the foundation of the rock and the rock stays put. You can see the dry rock which is quite white. Now that it's been running for over a year you can't tell what was live and what was dry.
Well crap! I went to Reef Culture with my wife today to get base rock. It's $3.99 a pound and they give you 5% off if you have one of their t-shirts. We picked out some very nice rocks. More than one of them was 20 pounds by itself. Just massive rocks.
I ended up getting 80.25 pounds so just over $320 worth of rock.
My tank swallowed it!
I am guessing I have another 100 pounds or so between the two tanks I will be taking down but those are mostly smaller pieces (comparatively anyway). The idea of getting this beast was to have lots of open space but this is a bit ridiculous. I don't really want to spend another $200 on rock but I don't know if I can even lay this out how I want it. Aquascape is huge with me and I am really picky...I'm also slowly eating away my tank budget.
I posted above how I tried to do the rock and just gave up. I went and purchased some more rock and it turned out to be just the pieces I needed!
I have to give huge credit to my wife who got her hands wet and helped out with this.
The first photo is a whole tank shot to show the stack and the second photo is a close up of the stack. It's a little hard to see in the photo but the flat pieces stick out and form not only a great overhand but a great shelf. The top corner by the over flow is designed so that a giant paly rock I have will fit right there. The rock is huge and covered with palythoa and will be a great topper or that stack. On the other end there will be some acropora and my corcea goes in the dimple in the rock toward the bottom of the stack.
As you can see this is only one half of the rock work. I have the 50 pounds coming in and what I have from my other two tanks that will make up the right side of the tank and some extra areas.My Marco rock came tonight! How very exciting for me. This is the first time I have ever gotten rock online so my expectations were reserved at best.
I was very impressed with what I received. I had requested three large rocks and got them.
The first photo is what I saw when I opened the box. I removed the top bubble wrap to find each and every rock was individually wrapped. That really impressed me. I am not quite sure how I expected it to come, but this was certainly carefully done.
The next photo is the first rock I pulled out, then the next is all 50 pounds on the floor. They stated on their site they add 4 pounds to make up for rubble so the last photo is of the rubble that was left once it was all taken out. Certainly not 4 pounds worth. This is especially surprising since the rock is very fragile and snaps off easily.
The tank has rock from four local pet stores, Marco and one rock my wife brought me from a trip to the Bahamas in it! I feel like a grape picker for a winery...only the finest rock goes into our reef Chablis!
Anyway...I still have rock in my two tanks that I will use to complete this aquascape but the main design is set. Certain areas will be hidden by corals of course and others will have their lines completed by rocks that exist in my other tanks. For example a giant rock of green paly has a specific place it wants to go.
It's difficult to see the depth of the rockwork from photos. The two sides don't actually touch each other and it goes back into a canyon, lots of depth and 3Dishness to it in real life. In this photo it looks like one solid wall but it isn't.
It's been a little while since I posted an update, that's because I've been working. The rockwork is all done and I was able to move everything out of my 50 gallon. It was a huge job! I broke a couple of SPS but nothing major to speak of.
Picture one is sort of a full tank shot. I've only got one light on the tank until the LED's arrive. Actually the lights arrived but the hanging kit has not so I can't put them up.
The second and third photos are pretty cool to me. There is a rock I've had forever and it ended up fitting perfect on the end of the new rockwork. When I say perfectly it's insane...there was a dip in one rock and an outcropping on another that fit as if they were made to click into each other.
The next photo shows everything just piled in, nothing is in it's final home yet because I still have to tackle the BioCube and the corals in it. The last photo is of a mini-brittle star (it was pretty big for a mini) and I took it just to show how bringing the rock over from the old tank instantly makes the new tank alive.
All the fish are in the new tank and very happy. The tang and dottyback used to spar but now there is so much room they don't care anymore.
I added a few extra hermits, peppermints, and snails.
Everything lived, catching the fish was pretty easy with the exception of my watchman goby who resisted. He was exhausted. I do not plan on keeping him and the pistol shrimp but am having second thoughts since my daughter expressed a desire for me to keep them.
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