(Sorry to double post, but it looks like I can't edit my posts for some reason)

Okay, he left the water bowl, and I saw little black particles at the bottom (the biggest were perhaps 1 millimeter in diameter). I took a picture, sorry for the quality but it's not easy since they're so small. Also, I change the water everyday and since I use paper towels, these can't be substrate or just dust.

http://uppix.net/b/e/9/2589136f0f451...231d01ef5c.jpg

Do you think these could be mites after all? (I of course made a deep cleaning of the tank and everything in it, don't know if this will be enough though)

Again, thank you for your help!