Having too many fish in a tank compared to the amount of water will in the end result in the death of some of those fish. It doesn't matter how clean you keep the water ,fish will die.
I once had a tank that was 100g and filled with so many fish(size ranging from 2" to 6") that there should have been traffic lights to ease the congestion. Now on this tank I had a 24h drip system replacing the whole tank every 24 hours, it also had a very big filtration system on it too. In this tank nitrates rarely got over 5 ppm ,ammonia and nitrite where always zero, so the water was in better shape than most tanks with very few fish and regular water changes. In this tank a strange thing always seemed to happen, the fish would slowly die off over a few months, so I would add more to replace them and it would happen again and again and always the fish die offs would stop until around a certain amout of fish remained. I then left it as was b/c I was sick of replacing them. They then lived a happily with only one or two fish dieing every now and again.
My point is that there are other factors involved that will affect the health of your fish, you may not believe it but fish actually are alive and they swim around b/c they have a thing called a nervous system and brain combined with bones, muscles and other living cells. Now just like you and I or even your cat or dog if those functions are stressed,disease and death is inevetable.
Why not learn from other people mistakes and either get rid of some of the fish or get a bigger tank or better yet buy another tank to the extra fish in.
