sick-looking cichlids

Question
2 weeks ago I did a 25% water change on my 4ft cichlid tank (which contained about 30 juvenile African cichlids and 10 baby bristlenose and has been running for about 3 months) after being on holidays for 3 weeks. When treating the water I used stress-coat (which I've never done before), natural trace and cichlid salts (all of which are API products). The next day my husband discovered 17 cichlids dead and the rest, including the bristlenose, were all hovering near the top of the tank. By the next day, the remaining cichlids seemed to be fine and there were no more deaths. We put it down to the fact that the temperature was too high (we're nearing summer and live in Australia) and turned off all the heaters.
I bought 15 juvenile cichlids on Friday and did another 25% water change on Sunday, using the same products as before. By Sunday night all cichlids were ignoring their food (something they've never done before) and were all hovering near the top of the tank. When I woke this morning (Monday) they were still hovering near the top and one was dead.
The only difference that I can see between these water changes and the previous ones is the use of stress coat for neutralising the water. Is that what I'm doing wrong? I did a series of tests of the tank about an hour after the water was changed yesterday and had 0.25ppm ammonia level, 7.6 pH, 0 nitrite and 40 nitrate. I understand that the ammonia level is too high - are chemicals the best way to fix that?

Answer
Hi Leah,
  The best way to deal with high ammonia is to do a series of water changes.   The fact that the fish were near the top of the tank suggests a lack of oxygen, which can be aggravated from high temperature.  Do you have a good source of air going into the tank?

-- Ron C.
  rcoleman@cichlidresearch.com
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