What were your Nitrite, Nitrate, Phosphate readings? More to water quallity than ammonia and clarity. Ammonia is broken down by benefitial bacteria into Nitrite and then it is broken down further into Nitrate. Nitrates should be no more than 20ppm, nitrates can only be removed by waterchanges, live plants do use nitrate for fertilizers. Goldfish as said before are EXTRMELY messy. We have a 320G goldfish tank that only has 10 golds in it, soon to add 6 more to it and thats it. 20G each is fine. Also the larger your tank the more stable its water conditions will be. 3 goldfish would do wonderfully in a 50G tank and it would be alot easier to keep stable than a smaller tank. IMO the "tumors" you had were probably a result of a water quality problem, really just about any disease in a fish is the restult of water quality going wrong. I am very sorry for your lose and hope that you get the fish situation fixed. Not that I'm telling you that you MUST, but am asking you to do this because fish are creatures just like dogs or cats and must have adequate space and certain requirements in order to survive. Think of a fish tank as a .......ummmm...hamster cage. The more hamsters you have in there the more waste produced, the smaller the cage the less space they have so the waste piles up. Eventually if you have 3 fully grown hamsters in say the equivalent of a 5G tank, no matter if you clean that cage every week it will still be polluted and they will die. The only key difference in this analogy is that.....water doesn't show its "polutants" like say shavings does. What looks to be a nice healthy tank can actually have horrid amounts of waste in the water. Anyway, I am getting off my soapbox and this fish activist is gonna go now.LOL!
