Hi there, sorry to hear you're having problems: let us know how things go and if they improve.
Here are some recommendations:
1. Turn off your lights asap, and leave them off until you can get that issue resolved. If they are creating too much heat, you are best with them off. Fish do not need so much light as a flourescent gives off (it's primarily for viewing, to give them a sense of day and night, and for plants).
2. I wouldn't drop ice cubes directly into your tank as was suggested: you might prefer to float a bottle of frozen water in the top instead. It will work for longer, will avoid exposing your fish to tap water, and will provide a gradual change in temperature as opposed to a sudden one. I had to do this in the recent heatwave too, and it worked just fine (my temp went up to 30c in one tank at one point and I reduced it to 28c with this method).
3. I wouldn't change so much water all in one go: the changes in condition will stress your fish. You are best changing 20% on day one, and then another 20% on day two rather than 40% in one go. But, you do need to change water out just in case of contamination as people have said. Don't put cold water in, thinking it will help reduce temperature. It will only shock them further. Make it
slightly cooler than it currently is. Every water change you do, reduce the temperature slightly so it's a gradual process. If you are only doing weekly water changes at the moment I would up it to twice weekly anyway for now.
4. Add an airstone/bubble disc. As has been said: higher temperature means less dissolved oxygen in the water. This will not be helping your fish at all. Adding an airstone will add more oxygen.
Remedial issues:
1. Where are your tanks positioned in the room, that might explain why they are getting so hot? And, how big are they? I would seriously consider moving them to a cooler spot if that's at all possible (I know moving tanks is a real tough one, but it can be done: I've just helped someone move one 3' and two 4' tanks to make way for a 6' and while it's back breaking it's not impossible if you make a clear plan and do it with common sense: i.e. emptying tanks and so on!).
2. What light fitting/tubes are you using? Are you using halogens?! What size tank do you have, and what is the wattage output of your bulb/tube? The smaller the tank, the less output you need...I'd really recommend replacing your lighting system if it's causing such increases in temperature.
3. You are best using the ye olde fashioned glass thermometers. They work better than digital, and are safer than probe therms. If it 'aint broke don't fix it they say, and glass therms have never ever caused me a problem in all the years I've been using 'em
