Choosing Compatible Fish
How to Figure Out Which Fish Can Be Kept Together
Choosing compatible fish for your aquarium is actually quite simple, however, it does take some research. When looking for compatible fish, there are three basic rules to keep in mind: size of the fish, temperament of the fish, and the fish's care requirements.
Size of the Fish
Generally speaking, a fish will eat anything that it thinks it can get in its mouth. In most cases, this even includes the fish's own young! Though some fish do exhibit parental concern, many lay their eggs or have their babies, and then the babies are on their own.
With this in mind, remember that a fish that will get large will be likely to eat fish that stay small, even if the large fish is generally considered 'peaceful.' This isn't because the fish necessarily get mean as they grow, but rather that they start seeing the little wriggly thing (which may be your pride and joy, or even their own offspring) as a tempting treat.
Temperament of the Fish
Keeping aggressive fish with peaceful fish is often a recipe for disaster. Even if they are the same size - or if the aggressive fish is smaller - the aggressive fish may tear and pick at the less aggressive fish until the victim becomes less able to resist disease or dies!
Sometimes, an aggressive fish will keep more peaceful tank mates away from food or ideal hiding places, causing the friendlier fish to die from malnutrition or become more disease prone due to stress.
Also, keeping a peaceful fish with an aggressive fish will leave the more peaceful fish looking rather torn up, even if the aggression between the fish is not sufficient to cause premature death.
The Fish's Care Requirements
When looking for fish to keep together, bear the care requirements of the fish in mind.
Aquarium Temperature and Salinity
Keeping fish with substantially different temperature or salinity requirements together will often result in one of the fish having problems. Warmer water carries less dissolved oxygen, and therefore fish that require cooler water may be oxygen deprived or even suffocate in warmer water. Similarly, keeping fish that require some salt in their water in freshwater can cause skin, gill, and digestive system damage as the cells in this exposed tissue rupture from taking in too much water. Conversely, keeping fish that require fresh water in salted water can cause the fish's exposed skin and gills to contain less water than they should, making breathing difficult. Either of these conditions will increase the fish's stress level and disease susceptibility, and can cause discoloration.
Aquarium Hardness, pH and Alkalinity
Similar problems may manifest if fish that require hard or alkaline water are kept in very soft or very acid water and vice versa. Remember, however, that all fish live in dynamic environments where the water temperature, salinity, and chemistry change over time, and that the conditions that a fish will tolerate are often very wide. Most fish are just fine over a range of a full point or more of pH, five to ten degrees of hardness, or a tenth of a point of specific gravity. Though there are exceptions, the fish that cannot tolerate a range of water conditions are just that - exceptions - not the rule. The most dangerous thing that can happen with water conditions are sudden and drastic changes - taking a fish out of a nice warm tank and placing it in a bucket of ice water, or pulling him out of a marine tank and dropping him in fresh water can be devastating.
Another issue that can come up are specific feeding requirements some fish have. Most fish will do fine on a wide variety of foods, but some fish have very specific requirements as to the content of their diet or the frequency of their feeding. These fish may be starved or malnourished in a tank where they are not fed properly, or their feeding may become a problem for the other fish in the aquarium. Be aware of any special feeding requirements before you purchase a fish, and make sure that isn't going to cause problems in your fish tank.
Remember, when selecting fish for compatibility, consider the fish's size, temperament, and care requirements. This information is available in many books about fish keeping, as well as from many quality pet stores.