01-02-2010, 04:24 PM
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I agree with mollies on the size issue. But I also have a comment or two on an important aspect of your original question involving compatibility.
Fish come from different habitats, and many things make these places different. Water parameters is a significant part of this; your African rift lake cichlids are endemic to the rift lakes in Eastern Africa and are unique to that environment. The water in these lakes is very alkaline and hard, by comparison to many if not most other freshwater fish habitats. Nature has evolved these fish accordingly, and to keep them in a home aquarium in good/best health, the water should be as close as possible to what they are designed for.
Fish such as discus and angelfish, mentioned previously in this thread, come from completely different water; it could not be further apart. These fish live in streams that are very soft (less than one degree hardness compared to around 30 dGH for the rift lakes) and acidic (pH around 5-6 compared to 8+ in Lake Tanganyika). Their biological metabolism is simply not built for this. Plus, placed in an environment of rock and white sand with no plants would stress out discus and angelfish to the point of bringing on health problems and disease.
When considering fish for an aquarium, "compatibility" has three aspects:
1. Water parameters. Temperature, pH, hardness, salinity (brackish opposed to fresh). Fish are built to live within fairly specific confines. It is true that some fish can adapt quite a bit to differing water parameters, but I am not personally convinced that long-term this does not cause problems internally and lead to increased risk of disease, shorter life, etc. There can be no question though that providing the same, or reasonably same, conditions as in nature will be positive with respect to the fish's long-term health.
2. Environmental objects. Plants, wood, rocks, strong current or no current. Fish that are programmed by nature to live among tree branches and plants will, if placed in a relatively bare aquarium, feel constantly threatened; the fish does not "know" it is safe, it only knows that it has no hiding places and its natural instinct says it is vulnerable. The continual stress will weaken its immune system and bring on any number of problems and disease, and probably premature death.
3. Behaviour. Fish that are being harassed by bullies will be under stress, and bad health follows.
A successful "community" aquarium, by which we mean one containing more than one species of fish, must have fish that are relatively the same in water parameter requirements and environment, and then have behaviours which will not cause stress to the other fish.
I hope this helps to clarify things.