I have a 77 gallon fluval tank which is just over 4 years mature. I'm not a great expert in the species names whatsoever, I actually couldn't remember a single on if I tried. However, I'm quite sure some cross breeding has occurred. There are new fish in my tank and they look different than the rest. Some of these new fish are over a year old mind u so it is easy to tell that they don't look like the others. Any thoughts on this? Does it happen?

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Yes, hybridization can and does occur in the home aquarium. Especially if the number of males well exceed the number of females.