Thank you, this is a good site. I have a question, however about one thing that I see in local pet shops. I am afraid this is a silly question, but I can't find any info about it, so I'll go ahead and ask.
Well, there's this type of very porous, rough to touch piece of rock that looks nasty as well. It might be decorative in a tank though. It is so rough that I am afraid it might hurt the cats. It should be some volcanic stuff unless it is something artificial. Which types of tanks should it be used for? Maybe it is meant for marine aquariums? I am so ignorant