Grain Mites, a close relative of dust mites, strive in hot and humid condition (temperature above 71 degree F and humidity above 50%). They cannot survive the condition of humidity below 50% for extended period of time. The aquarium seem to be the perfect breeding place for them. The heater and water create water vapor from the aquarium maintain warm and humid condition. If aquarium food is spilled and left uncleaned, it also provide food for the dust mites. With the optimum temperature, humidity and food source, it seems to create a perfect breeding place for dust mites.
To my surprise, I found a grain mite alive in the water collected from my aquarium!
I collect some more samples from the aquarium hood. I found more some more trace of grain mites: Some broken legs and a dead body. To collect the sample, I simply use a 3M tape with the sticky side down. I grab both ends of the tape then gently smear it on the surface of the hood, then tape it over a slide. For observation, the specimen was placed on the mechanical stage upside down.