I am very new to keeping fish an aquarium was given to me it came with a couple fish and we kept the same water that had been in it for years. But I think I may have overcrowded it. Its a 30 gallon I have 6 tiger barbs 5 harlequins 3 corys and 7 red eyed tetras I have never done a water change until this weekend I did a 50 % I didn't even know you had to do that but I do now I was given little direction when I got it so I'm learning on the fly so any advice would be greatly appreciated so i can keep my fish healthy