Well, since it's summer - I am kind of worried about my aquariums becoming too ht. Since I live in a basement, it's probably not bad. And right now it's night, the 20 gallon aquarium is around 72 F. But during the day sometimes it's up to 75 F . I think a good tempeture for goldfish is 78 G but I'm not sure. That's almost as warm as the air here! 80 F here and if my aquariums were 78 it would only be two degrees less. Anyway, so I'm wondering sould I put a fan on (- or add ice cubes) for the water to cool down or is it just fine? When I put my hand it, it's warmer then what it is in spring or a few weeks ago.

I dunno, I know they get cranky above 80F. It was recently 95F in my house (I have no AC available), and my goldfish started splashing a lot at me, kinda like yelling at me to turn down the heat or something. I set the temperature usually at 76F in my tank. I was thinking of ice cubes in a bottle, but that would only last for a few hours, if even that much, and the swing of temperature down and up probably isn't sog reat either.

Jasmin the mid 70's is what you should aim for. Obvisiouly if it gets alot hotter in your area you could have some soda bottles rinsed out and filled with water and freeze them.
That way when you have to add them it doesn't change the chemistry of the water, and you don't have to add a conditioner.

I have a couple frozen "just in case". I would say though be careful if it gets too hot and you cool it quickly you'll do more harm than good.

Good luck.

Its been hot here in SO CAL too. My 100 gal. was a steady 83 degrees. So I decided to hook up my ¼ H.P. Aqua Logic Chiller for the first time. I’ve had it for over a year and it was collecting dust in my garage, so it was way over due. Anyway I got it used and it came with a Medusa duel controller. Long story short it works like a dream. I set the target temp at 77 degrees, and there it sits rock solid. It beats leaving the a/c on 24/7. If anyone has been considering a chiller, I say go for it.