Will a 30 gallon tank generate enough algae to support either a group of Otos or a Farlowella? How important is it to have an algae eater? What do you mean by problem algae?
Essjay referenced some of this, and I agree. There are two approaches to so-called algae-eating fish. The wrong approach is to think they will solve problem algae--they will not. The correct approach is to balance the light and nutrients for the plant species so algae never becomes problematic, and then you can control it with specific fish if the species suit the aquarium ecosystem. All fish have their own specific requirements that must be provided for if they are to be healthy.
Algae is perfectly natural in every aquarium, and it is impossible to eradicate it; if you did manage to, you would not have a healthy aquarium at all, quite the opposite. Algae needs nutrients, and there are plenty in a tank with fish. And it needs light, and any light will suffice; unlike higher plants, algae is not at all fussy and can manage in any situation. Green and red algae occurs in light, but if there is insufficient light for these, brown algae (called diatoms) will appear. So expect algae of some sort, always. The aim in planted tanks is to keep it under control. In tanks without plants, it actually is beneficial because like higher plants it uses nutrients in the presence of light to produce oxygen, and that is not "bad" at all. But if out of control, it can appear messy, and after that, become a real problem as it indicates a very unhealthy (for fish) system. Cyanobacteria which is not an algae but a true bacteria can be terrible, and green water (which is an unicellular algae in the water column, turning it green) is also not something you want.
"Problem" algae is algae like black brush/beard (there are several other types/species) that can literally destroy a planted tank in weeks if it increases rapidly. Problem algae is caused by an imbalance in the light/nutrient area, always; there is nothing else that causes problem algae. If the light is too intense, or not intense enough for the plant species, the plants will struggle and algae has the advantage because it can "manage" with anything. Each plant species has a light requirement range, important because light drives photosynthesis, so if it is not within the range for that species, the plant cannot photosynthesize properly. And if it is not able to photosynthesize, it cannot use the nutrients, so these then feed algae, and maybe worse (the cyanobacteria, etc).
No fish is going to handle problem algae, with a couple exceptions. But these fish have their own specific requirements I won't get into, and they are usually not suitable for the average tanks. So you control the algae yourself, with a balance of light and nutrients.
Common green algae will appear on every surface covered by water, as part of the biofilm. This collection of various bacteria, algae and microscopic critters is an essential part of a healthy aquarium, just as it is in nature. If you feel a chunk of wood or plant leaf, even the tank glass inside, it is usually sort of slippery, or slimy but in a good sense. This is the biofilm. Some fish species will graze this, eating much of the living organisms including algae. Mollies love this, as do many of the African rift lake cichlid species. But there are some very specialized feeders, the Otocinclus, Farlowella, Bristlenose pleco, and some others. It can be beneficial to have one of these (meaning, one species, as combining species is usually not a good idea, simply because they are competing for the same food, and being so effective they can deplete this quite successfully), and they will readily "learn" to use algae or kelp-based foods to supplement their natural food. I have raised several batches of
Farlowella vitatta fry, and it always amazes me just how rapidly they learn what those round disks are that appear once a day; within literally a few weeks, they will converge on these within a couple of minutes after I drop them in. The rest of the day they spend grazing every surface, from the smallest plant leaf to chunks of wood, rock, the tank walls, filter tubes, even the heaters. This is beneficial, as it helps keep this under control. And the fish often have interesting behaviours that make them nice additions to a community tank.
I would however let the tank establish for a few months before adding any of these; they settle in much better when natural food is present to start them off.
I looked at Farlowella, but it seems they are fairly hard to raise as well.
Not really difficult, but they have requirements. Soft water is essential (you have that, with GH at 20 ppm, it is very soft). Acidic pH is also beneficial, and without getting into the chemistry I would suggest that you will likely see it lower below 7 once the tank's biological system gets stabilized. With such a low GH, the pH of your tap water is probably due to something being added by the water authority to raise it. I have this in Vancouver; depending what they use, it can dissipate out quickly, assuming the KH (carbonate hardness or Alkalinity) is also low like the GH..
Second, they are sedate fish, and do not appreciate being pestered by other fish. They do not like moving environments either; when I have had to move them to a different tank, they usually let me know they are displeased. Cories are like this too, as are loaches.
Provided you introduce them to an established tank, and provide an algae-based sinking food a couple times a week, they should settle in nicely. They also love grazing dried leaves, like oak, maple, beech. I collect these in the back garden every autumn, and use them in some tanks as part of the decor (leaves litter most streams of tropical areas) but also as important first food for fry; infusoria will be produced on the leaf surface as it slowly breaks down and fry will greedily graze this; fry grow much faster with dried leaves in the tank than otherwise.
