EP13B said:
ok my nitrie levels are at 0 as well.
any ideas
I hate mysteries!
Anyway, what I'd do in your situation is immediately institute a regime of frequent partial wate changes (10% every few days).
I'd also test for nitrates - nitrates are generally considered harmless but the concentration can build up (particularly if you're water comes from a farming area and is contaminated with nitrates). Over 25ppm nitrates could weaken the immune system of your fish.
Also check for "dead spots" in your gravel (ie anaerobic bacteria) - that is, clean your tank and make sure you clean the gravel thoroughly under rocks, bogwood etc. If you notice any bad smells (sulphur dioxide), do an immediate 100% water change and preferably move all your fish into a holding tank with 50% of the used tank water and your filter (no decorations). Also, if that is the case, get back to us because its a tank emergency.
Check temperature and pH. If pH is low, check KH just in case your water is too soft to maintain a steady pH. If the temperature is a little high be aware that warm water holds less oxygen. However, angelfish can go near the surface where the oxygen is highest and in that case, I'd expect your bottom-feeders to die first. It's weird that your gouramis were among the first to go.
Have you got CO2 in your tank? If so, check your CO2 levels.
If none of that is the possible cause, we need to consider other possibilities. Has anyone cleaned the tank glass with a glass cleaner of some description? Has there been painting, crop spraying or pest spraying in your area or in your house? Do you use air-freshener? Have you checked your house for carbon monoxide? Have you sprayed a cat or dog with flea spray anywhere near the tank? Has anyone been using housemite insecticides on the carpet? I had a friend lose every single fish in her marine tank because her cleaning lady cleaned the front glass with Windolene and the fumes dissolved into the water. We only figured it out when she was cleaning up the remains and we kept noticing this strange chemical smell.
If your Sherlock Holmes'ing draws a blank on toxins, you'll have to consider disease. I'd keep the water absolutely pristine and use a bit of Melafix tonic in the water - and pray.
All the best - do tell us what happens.