We now have had more than 2 weeks of weather that should be called summer in stead of spring. As a result the temp of my big tank is now 26°C. And I noticed that as a result my fish seem to nap in the hottest hours (between 2 and 5 in the afternoon). Is this normal behaviour?

How hot may the tankwater become actually for goldfish?

We are going to get a big storm on thursday so the temps will go down to normal springtemps (16-18°c), so the tank will be cooler in a couple of days.

26C degrees is pretty average for my tanks in summer, sometimes they go as high as 28C. Goldfish can be kept at temps of up to 30C but that is usually only done for medical treatment purposes. My goldfish do not get lethargic in the warmer temperatures but they are used to it, in fact fish are cold blooded so they usually get lethargic when the water drops below 10C.

There is less oxygen available in warmer water, so if you think your fish are acting lethargic you could add an air stone or take the lid off the tank and direct an air flow over the surface of the water with a fan.

I'm not sure how big your tank is but if it is 25 gallons plus and has a lid the temperature should not drop lower than room temperature. Also water changes temperature much slower than the air around it.

Is it possible that they remember from when they lived in a dirty pond, that on hot days there isn't enough oxigen, so they do not move much?

They have an airstone in the 70 gal and I lowered the water level a bit for more splatch. To prevent Gigi to even think of airsucking (she's my little problem fishy).

Lethargic is not the best word, it looks more like they adjusted their sleeping pattren. 3 weeks ago they would already be sound asleep round 9 in the afternoon. Now with their siesta they are not even asleep at half past 10.

After the storm last night, the tank temp went back to normal overnight. Lo and behold they didn't nap today . I 'm starting to think they are experimenting with me . Godzilla played salmon for a big part of the afternoon, she just loves to work out against the filter. After that she started with her other favorite pasttime, searching for small pieces of food she might had missed before.

my tanks recently have been in about the 26-27C range... and it's not even the hottest of summer yet!

i do notice that if it's warmer they kinda drift around and glide through the tank as opposed to actively swimming back and forth.
they do get more active in the mornings and nights when the temperatures aren't as warm though.

When the water gets that warm, adding an extra air feature as Fishmerized suggested is a VERY WISE thing to do. Warm water holds substantially less oxygen than cooler water. This impacts the fish's ability to "breathe" and it also will impact the efficiancy of the biological filtration - the beneficial bacteria need oxygen to create the nitrogen cycle.

Coolling the water - or adding in a whole bunch more air - by circulating it to the surface with air features where it can pick up more oxygen - is a good idea. I just read where someone questioned the lower level of the water in a tank- this would be one of the times where it might be a good idea. More splashing = more oxygen. Open the tank lid. Set a fan going across the water surface. Oxygenate that water and your fish will be happier.

I did lower the water level to get more splashing on day 2 or 3 of the hot wether, I didn't notice any difference though. Now it is a lot cooler I've raised the waterlevel again.
Opening the tank lid is not the best option, as my tank stands in the garage, where my fathers workbench stands right beside it, a lot of stuff (sawdust, doghair, etc)could get inside, I'm scared it might block the filter or the fish, I don't think they could digest either doghair or sawdust.

In 2 months I will be able to open the lid on hot days, because we should have moved to our own place by then