Which are suitable sand grain sizes?
I have some questions as I will use sand as bottom material in my new aquarium: The sand I will be using has about 0.5 mm grain size. Is this too fine?
no, that is alright. However, it must be round grained (not crushed sand), especially if there are fish in the aquarium that live on the bottom or even burrow in it.
There is some other sand with 1 - 2 mm grain size, which would be suited best?
Both are suitable (however, 2 mm grain size is actually fine gravel rather than sand), I recommend plenty of trumpet snails in both cases.
Isn't it possible that rotting sludge occurs when using too fine sand (how does rotting sludge form?)?
Quite on the contrary: The risk is considerably lower than in case of coarser material, because organic matter (e.g. possibly uneaten food) does not sink into the gravel and rot at all (this is how rotting sludge forms) but remains at the surface. The above mentioned trumpet snails furthermore loosen the bottom, thus reducing the risk of rot formation.
Please also refer to the FAQ "What do I have to consider when using sand as bottom material"; also under "General"!
Dr. Bodo Schnell