how do you keep your fish tank cold enough for your fish??

How do you know it's heating up? Do you have a thermometer in the aquarium? What kind of fish are they? And if you have measured the temperature what is it?

Normally, you shouldn't buy fish that require temperatures below what you can easily provide. It's a lot easier to heat it up a bit than to cool it down.

well i don't have a thermomerter in my tank but i think it's heating up because the fish just seem to huddle up in one corner of the tank( i think thats the colest bit of the tank) and dont seem to move around much!

they are goldfish! is this behaviour caused by temperature changes in the water? to warm for the fish?? it's summer here so outside gets up to about 29 to 30 degreese.

thanks.

You should get a thermometer for your tank, they are very cheap (a couple dollars) and will let you know whether heat is an issue more accurately. Avoid the sticker types, that you put on the outside of the glass as those are inaccurate.

As long as the water is not over 28 degrees they should be fine.

Years ago I had some outdoor goldfish in an outside pond. They seemed content all year round whether it be 100F in the summer or below freezing in the winter. Winter months the pond would freeze over but come spring they'd be swimming as always. Maybe have the water checked to make sure it's alkaline enough for them.

Fish generally hover close to the top of the tank for oxygen reasons... not temperature reasons.

The water in the tank does not fluctuate more then the temperature of your own home . Heat/cold changes in a tank are normal and something to be expected and can be tolerated by many fish.
Goldfish especially.

Do you have any kind of bubbler? Anything that makes the water move ?
Any live plants?

Goldfish are odd fish to begin with. They can sometimes stay in just one area for a while.
What else are they doing? are they swimming upside down?

My suggestion, if you dont already have something, is to get the top of the water moving. A bubbler ( under 5.00) would be enough. But goldfish are terribly acidic.. They put out ammonia that even they cant handle after a while and you will need some kind of filtration in that tank for them to survive for a long period of time.
Good news is that goldfish thrive in what other fish would consider dirty water.. so you dont have to have a super powerful filtration set up.
You could get away with a 10 gallon bio filter system 20.00$ or so, and have a good, healthy , oxygenated tank for them .