Summer is approaching again. Last summer, I had a lot of trouble keeping the temperature in my aquarium at 30°C. My fish prefer 21-24°C, but I couldn't get it any lower than 30. So I want to avoid having to go through all that trouble again, and to keep the fish happy. I doubt fans are going to help enough, and I don't want to have a refrigerator or an A/C in the room. So I figured an alternative solution which involves passing the water through a heat sink (like the ones on a CPU). Heat sinks are usually made of aluminum. So I was wondering whether running water over aluminum could be harmful.
Aluminum is bad in saltwater tanks which I'm assuming is what you're running because freshwater tanks are generally fine at 30 °C.
I honestly don't recommend any metal in the tank or a component in contact with the water unless it's titanium. Aluminum gets degraded fairly quickly and even metals that are more suitable for saltwater, such as bronze, have high levels of copper in them which is not good for aquariums.
Additionally, CPU cooling systems are not designed to cool components to a super low level unless you're talking about active cooling systems. They work great for computers, but computers don't need their operating temperatures to be as cooled down to as low as 30 °C. And, even with an exceptionally designed heatsink, you're still at the mercy of ambient temperature, and the closer you get to it, the less efficient the heatsink will be.
Short of using an active cooling system like a chiller or cooling down the room that the tank is in, evaporative cooling in a low humidity environment is going to be the best and cheapest way to get the tank's temperature down if that applies to you. If it doesn't I would be looking at cooling the fish tank room, a chiller, or a Peltier module based cooling device if it's a small tank.
Depending on the fish, 30 °C is generally OK if they have air/oxygen. The higher temperatures reduce the solubility of gasses in water, in particular oxygen. So an aerator and/or a filter that moves the surface water, such as a HOB type, is good and should be enough. A small fan blowing on the surface could help.
I have a tank on the deck ,in full shade, that gets over 32 °C , but it only has anabantoids (paradise fish) air breathers like bettas. Otherwise, I have many variety s of tropical fish that get at least to 30 °C.