I believe it is damage due to salts, perhaps from fertilizer. Flush with water three times. Make sure you are adding the correct amount of fertilizer. Has the water been treated with a water softener?

I really don't use much fertilizer, and only started applying it previous month and the leaves were alreay getting brown before I started fertilizing. And yes I used softened water because I read that when you have hard tap water it is not very good for the plants because it makes the soil too alkaline.
I have searched the internet and it could be caused by low humidity (which is possible because they are near a radiator).
And one of my peper seedlings had a tiny worm in the soil and another citrus tree (given by a friend) has mealybug. Can it be caused by one of those? And what should I do?

Collect rainwater from your downspout or street gutter but don't use soft water. There will be to much salt in it. It will kill your plant

Oh ok, thanks.
The only problem is I don't really have access to rainwater, I live at a small appartment..

Do not use water that has been treated by a water softener system because it will kill your tree. The softened water is causing the damage to your tree.

And normal tap water, is it ok? Or can I do something to make it good for my plants?
My trees will not die if I stop using softened water, right?

Tapwater that has not been treated by a water softener is OK. Your tree should not die if you stop using softened water. Remember to flush with untreated water as I described above.

Irrigation water that has been softened via a salt - is just that, a salty water. Salt water will desiccate and kill the thing quickly.

Thank you all for the answers!
I flushed the trees today with untreated water, and I'll stop using softened water. I did this because I read it was to alkaline for those trees, what can I do to make the soil more acidic when using alkaline water?

Get a cheap pH testing kit - the kind used to test the pH of swimming
pool or aquarium water. If your tap water is above pH 6.5, add enough vinegar
to adjust it to about pH 5.5 to 6.5. You will only have to add give or take 1 Tb/gal.

Thanks! I'll do it as soon as possible! I really love my trees and I really don't want them to die, so I hope this will help :)