Beginner Help!

Question

My Fish clumped up   Hello Expert Jaymie Johnson!
I own a 20 gallon fish tank in which I have 8 fish total. 6 goldfishes of which 3 are bigger than the rest and a small Koi Fish and a small plecostomus. They swim fine and their bodies look healthy. But they tend to group on the right hand bottom corner a whole lot. I wonder if they need more hiding places or a bigger tank. I partly change their water regularly and completely every month in a half or so. I am a beginner and I do what I find online. I have no good idea where to find reliable information about fish care. I find random sites. If I could have your honest opinion and a hand to be a better fish keeper I'd very much appreciate it.

Answer
Let me start by saying the tank looks good. The 20 gallon tank is not big enough for the fish you have. You also have a lot of decorations and that is a problem too, and I will explain. A 20 gallon tank, will hold 20 gallons of water if there is nothing else in it. When you add gravel, that takes between 1 1/2 to 3 gallon of water space depending on how much gravel you put in the tank. Now your 20 gallon tank will gold approximately 17 gallons of water. You have a lot of decorations, so now you have to subtract approximately 4-5 gallons of water space, making your tank 12 gallons. Once the filter is added, that removes another gallon making your tank 11 gallons. Now your fish are swimming in approximately 11 gallons of water. Some experts say that one goldfish needs a minimum of 20 gallons, with 10 gallons for every added fish. This means that in order to have 6 goldfish, the minimum size tank is 70 gallons. Goldfish can get quite large including the common, or comet. Koi should never be in aquariums since they are a pond fish and can also get quite large. The goldfish and the Koi are cold water fish, where the pleco is a tropical fish. This means the goldfish do not need a heater but the pleco does, therefore, it should not be in the tank with the goldfish. Your pleco should also be in a much larger tank. It can grow to 12-18 inches and sometimes larger. It should be in a tank no smaller than 80 gallons but 100+ is better. There are plecos out there that do not get as big. Doing a complete water change is also not good. In an established tank, there is beneficial bacteria that both the filter and the fish need to stay healthy. When you completely clean the tank, you remove all the bacteria, which in turn, recycle's the tank. Only do your 25% weekly water changes. Since the tank is too small, you can either do larger water changes every week, or you can do the 25% water changes twice a week.