Yes, most definitely I would check all levels in your tank. If you don't have a test kit, go buy one! That should be the first thing that you buy. But, I have to disagree with ohiorn67, it doesn't matter how big the fish is, I have seen this posted time and time again, but no one listens!!! It is the type of fish that it is. While it might "fit" in the tank, it has to have more water area to swim in than a 55 gallon can provide! It will get stressed, it will get sick, and it will eventually die. I know some people don't care and if it will live for a few months that is all that matters. So, if that is the case, have fun. But if you are trying to get a pet that you can keep, then ~ check your water parameters, fix any problems, trade the yellow in for something that would be comfortable in that sized tank! Trust me, there is a whole gamut of fish that would do great in a 55 gallon! Also, research to make sure that all inhabitants are compatible with each other before purchasing. It will save a lot of headaches later on!