When I was adding the tap water to the bucket to do the water change I made one big and fatal error. I added the water conditioner to the bucket first, then poured in the water. I thought it would mix up the conditioner better. WRONG!!!!!
The water conditioner reacts immediately to the new water. So by the time my bucket was 1/3 full, the conditioner was no longer working and the remaining 2/3 of the water was pure tap water. The water conditioner needs to be added after the tap water is in the bucket in order to dechlorinate all the water. The chlorine in the water I was using to change the tank was eating away at the slime coating of the fish. That's what the white stringy stuff is that has been floating through the tank.
This makes so much sense since both tank losses were after the water change.
Problem # 3
I am not using activated carbon in my filter. Sponges only. I received an e-mail from the water supplier and there is aluminum sulphate and chloramine in the water. Not too good for fish.
This sounds wrong to me. The water conditioner will react with the chlorine and chloramine in your water. If you only add a little water, it only contains a little chlorine. So you'll be left with some 'spare' conditioner. As you add more water, it comes into contact with more chlorine, reacts with it, and so on. If you added the water first, then the conditioner, the conditioner would only come into contact with some of the water at first - it doesn't immediately disperse uniformly throughout the water - it would be just the same. I don't think this is at all the cause of your problems - I would file this under 'dodgy advice from LFSs', alongside 'the fish will only grow to the size of the tank' and 'golden algae eaters are ideal community fish'.
I normally add the conditioner first and haven't (yet!!!) had any fish deaths. My water contains chlorine and chloramine. A number of people add the conditioner to the tank before filling up from a hose, also without problems. Make sure you're using a conditioner that neutralises chloramine as well as chlorine.
Also not sure that aluminium sulphate is likely to be a problem in the concentration that you normally find in tap water. People even add aluminium sulphate (e.g. Phosclear alum) to ponds to control phosphates. Most people don't use activated carbon except when they need to e.g. remove medications. Possibly your LFS is just trying to sell you something here?
I'd suggest that the problems are much more likely to have been some sort of disease, made worse by ammonia and also fluctuations in pH - you said the pH has changed between 6 and 8 - pH is a logarithmic scale, so 6 is 100 times 'more acid' than 8 - fish will tolerate a gradual change, but a large water change could swing the pH quite significantly.
Edit - Rabbut put it much better than me while I was cooking...
