Originally Posted by spike0544
How did you figure that out? Experience, or is there some type of formula? Will 50% water changes effect my biological filtration (bacteria)?
On the last question first, absolutely not. You could change 90% of the tank water every day--your fish would love that, though the constant disruption is unnecessary. Nitrifying bacteria colonize surfaces covered by water, and there are often more bacteria throughout the tank than in the filter. Every grain of substrate houses bacteria, every plant leaf, every piece of wood or rock, the tank walls, etc. As long as water moves over the surface bringing oxygen, bacteria will live on it.
To the first question: in a community aquarium, compatibility is the key issue. And this goes much farther than mere behaviour, though that is important. Compatible fish are those that share the same preferences/needs respecting water parameters (hardness, pH and temperature) and environment. Under the latter come issues like tank decor (plants, wood, rocks, type of substrate), water flow (the type of filter), light (some need dim light), and the activity level of other fish. When you match fish that have the same requirements in all these areas, you have fish that will be free of additional stress.
As an example: Putting a shoal of slow-swimming sedate discus or angelfish in with a group of very active swimmers like barbs is asking for trouble: the constant swimming of the barbs will annoy the discus or angels, causing stress. And stress weakens the fish's immune system, which means they are more susceptible to disease, parasites, etc. Sedate fish frequently have trouble eating or may even refuse to eat in such conditions. The tank size and fish-to-tank ratio really has nothing to do with this, though in very large tanks the sedate fish might find a quiet corner to hide in. But they will still be under stress. Fish send out signals to their own species and other species, pheromones and such, and fish under stress clearly signal that to the others in the tank which makes them even more stressed.
Once you select fish that have similar requirements, you can have more of them in a given space because they are going to be "relaxed" which means their tankmates will not bother them but will rather be complimentary so to speak. You still have to be careful not to overload the biological system. Live plants and regular significant water changes both help, but it is safer to maintain a reasonable fish stocking so as not to push the limits just in case.
But the point is that you can have more fish per volume if they are compatible in all aspects, as opposed to introducing fish that are "out of place." Taking a simple example: In a 30g aquarium, 8 Tiger Barb plus a few bottom fish will be the limit--but in a planted 30g tank you can have 10 Cardinal Tetra, 8 Hatchetfish, 9 -12 pencilfish, 10 corys, a Farlowella, and 8-9 Harlequin Rasbora and the community will be fine because these fish all share the same water parameters, filter flow (very slow), minimal light (just sufficient for the plants, with floating plants for further shade) and they are all sedate quiet fish--none are active swimmers.
This is why I and others always recommend, know your fish thoroughly before you acquire them. Not all fish will manage together in the same tank.