If you don't already have the 10w flourescent bulbs, switching to them can help. You could also focus some of those high power little desktop fans on the tank or get a room a/c unit or a room power fan unit for the window. There are tank chillers but they're expensive. You can add ice cubes and/or cold treated water when doing water changes, but that's kind of up & down on the temp.
I've never tried this so I don't know how well it will work, but it was just something I was trying to toy around with since this question has come up a few times. Maybe if there is anyone out there with these parts I will mention below that is willing to try this, we could see if this is a viable option. A chiller is really just the simple process of taking water from your tank, cooling it off, and then putting it back... except they want $250 for the cheapest one I've found. So the question is, can we do it cheaper.... maybe.
What you will need:
First you need a water pump and it would be nice if it didn't have to be a submerged pump so it doesn't have to be in the tank. Also it doesn't have to be really powerful, in fact the lower the power the slower the water will move through the pipe (you'll see why that's important in a second). I found one at the good Drs. website for $19.99 and it's the . The next thing you will need is a (have seen them for a few bucks at gas stations). Then the only other things you will need is some plastic tubing to fit the pump, a short piece of metal pipe with a large diameter that can fit into the cooler easily, some salt, and a bag of ice.
How to assemble:
1. Place your water pump near your tank and hook up some of the tubing to the tank. If you have a hang on back filter then just place the intake tube into the tank and make sure to cover the end with a straining device of some sort so the fish don't get sucked in. If you have a inline filter then you could possibly take the outflow of that and put it into the pump.
2. Place your Styrofoam cooler right next to the pump.
3. Find a screwdriver or something that you can poke a hole with that is as close to the size of the tubing as possible if not a bit smaller.
4. Take the lid off the cooler and poke 2 holes into it on opposite corners of the lid. (See diagram below)
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5. Get your metal pipe and spiral wrap your tubing around it as close as possible so the entire pipe is covered. Make sure to leave 2'-3' of tubing lose at one end and several feet of tubing lose at the other end. Also secure the tubing to the metal pipe so it doesn't unravel (some silicon glue or you can tie it down with fishing line... anything water proof).
6. Place the metal pipe into the water cooler and feed the 2'-3' line though one of the holes in the lid. Then take the longer end and feed it through the other hole in the lid. Make sure that you have some slack in the line so the lid can be removed but not have to lift the metal pipe out to do it. Secure the tubing in the holes with some silicon glue.
7. With the metal pipe in the cooler and the lid attached on top, connect the 2'-3' piece of tubing to the outflow of the water pump. Take the longer end and run it back up to your tank. If you have a hang on back filter then you could place it into the filter tank so that the water goes through filtration before going back into the tank, but if you took the outflow of your inline filter then just run it back like you always have.
8. Now that everything is hooked up, turn on the water pump and make sure that the flow of water is moving fine and that you don't have any leaks or other problems. And let it run for a little while to be certain.
9. Now open the lid of your Styrofoam cooler and pour in a little bit of ice. Just enough to cover the bottom of the cooler. Sprinkle some salt over all of the ice so it all gets a light dusting. Now add a little more ice and then more salt. Keep doing this until you have covered the metal pipe with ice.
10. Secure the lid back on top of the cooler and wait. The ice will melt and the salt will mix with the water (salt water gets colder then regular water) and slowly make the metal pipe really cold. This will help transfer the cold to the water that is flowing through the tubing and the slower the water goes through the tubing the longer it is exposed to this cool down process. Now (if this actually works ) the water should come back out of the cooler at a lower temperature and once it gets back into your tank it should eventually lower the temp of your tank water.
Note: You will need to keep adding ice over the course of the day and as the cooler gets full of water it will need to be emptied. Don't empty all of the water though because it will help keep the pipe cold when the new batch of ice is melting.
As I said, I have no clue if this will work. I've been looking at other options as well, but they all add up and make this too expensive to not just buy an already complete unit. Though another idea I had involved a closed loop system and instead of water, some sort of radiator fluid that gets colder faster. Then it would be built almost the same way, but instead of the fluid being emptied into the tank, the pipe would just hang into the water and cool it similar to the way your heater transfers heat to the water. The tube would have to be snaked up and down so it stays in your tank as long as it stays in the cooler. Another option (but it brings up the cost) is to use parts from a computer CPU cooling device. Instead of cooling a CPU though we would stick the cooling block into the tank. No clue if it's water proof, but I'm just kicking around ideas. The only problem is I just don't know about using radiator fluid near an aquarium. If it were to spring a leak and get into the tank it wouldn't be pretty. But it's an idea.
That cooler idea is really, quite inventive! I could imagine it working... though i haven't got room myself to try it right now. Well that, and it's not warm enough here just yet to try it. Though it would be really interesting to see another member work it to see if it works.