Major Smell

Got home last night and the smell from the fish tank was high!! I mean high. No dead fish but several gasping for air. I also noted they had not eaten the food I had put out in the m0rning. So mentioning this to Wanna, "Yes I fed them this morning", So did I chirped in Owen. Well they had had a good day in terms of food. Three times more than necessary.
Only problem is too much food is no good for them. I recalled that Travis in his manual stated if the water smells stop feeding. Done....
I removed the uneaten pellets and also the various leaves etc which had been added.
Later in the evening I decided to also remove the water. Yes the water. Pumped away about 80% and refilled with fresh tap water.
Seems to have worked OK as this morning the water was clearer and the fish still swimming, (the right way up!). I tried a few pellets but not much interest so will lay off the food for a few more days.