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In the Netherlands we have quite a big heat wave at the moment, which we're not used to at all. As a result not many people have air conditioners so our tanks can get quite hot.

Today it's going to be 37C (99F). My tank at 9 in the morning was 31C. How to prevent it from heating up further? I am not putting the lights on today to reduce heat. Will ice help at all, or is the volume of water too big? My tank is 160l, about 40 gallons. Water change will lower it by 1 or 2 degrees as it did yesterday.

Any advice will be appreciated

In the summer I set up tanks on a screened terrace. It has a roof and on three sides there are all screens attached to the exterior framing. The 4th wall is the house and the sliding door entrance. It is not uncommon during the summer for me to see temps in the range you indicate. And often I get extended periods of 34C-37C(93 -98F).

For the most part the fish in these tanks are Xingu plecos so they can take warm water. Bit I have other fish outside as well. The first things I do is open the lids, evaporation is my friend. When there is a breed this helps cool the tanks somewhat as well. When it is dead calm I will use a fan to get the air moving. And then I check the tank temps a few times a day. This way I can keep my tank temps at 86F (30C) or below.

Air movement and evaporation can work as long as you can open windows etc so the room is not hotter than the air outside. Next, shade helps. Tanks in direct sunlight in hot weather are very hard to cool.

Many of the ideas in the thread Shane linked can work. Years back I lived in a similar situation, no AC and a hot summer. We had a group owned tank in the apartment and it would get way to hot and so would the tank water. We devised a system using ice. We would remove water from the tank to a container (about a liter, the tank was 25 gal or so.), We had two such containers. These went into the freezer. We replaced the water removed from the tank for one container to leave some room. We usually put an "iceberg" into the tank twice a day. When one went in, the container was refilled and put back onto freeze. This worked pretty well. We always had another block of ice ready to go in if needed. By using tank water we knew it would be fish safe.

This method is fine for a limited number of tanks and ones which are not huge.

I am fortunate we have central air conditioning. Although it would be a major effort, if things got bad I could always move the fish back inside. But it would mean tanks in places they do not belong.