Hi there!
I struggled with the question of what substrate to choose when I set up my 50-gallon. After much research and reading, I decided on sand, for no reason other than I had become absolutely smitten with my cory cats and wanted to do anything necessary to make them happy.
I dragged my better half from store to store to find the perfect sand - not too light, not too dark, not too coarse, not to fine... I finally settled on Nature's Ocean black beach sand. Some observations/thoughts:
1. I am so very happy that I chose sand. I have spent hours just watching the catfish enjoy sifting through it, and their barbels have grown significantly.
2. From my research, I was very concerned about the substrate becoming anaerobic. I comb through it with my fingers at least weekly, but have never discovered pockets of gas or any signs of such a problem.
3. I have a couple live plants (soon to be more), and the plants have just taken off in the sand, despite the totally inadequate lighting, lack of fertilizer, etc.
4. I toyed with the idea of buying play sand; I read that it was dustier, and the play sand wound up not being the size (gradation) that I was looking for. I was committed to rinsing it well, and resigned to my tank being cloudy for a while (again, based on my research). I was happy that I purchased what I did, as the rinsing was much less effort, and the tank was cloudy for less than an hour - the sand is very evenly graded, so there's not nearly as much in the way of fines to cause such trouble. It settles down quickly, too, after my "combing" through it during cleanings, too.
5. If you have an acrylic tank, you'll need to be mindful of not getting any sand particles between the tank and whatever you're wiping it with.
6. I find that because of the ability of the cories to really work it over, there's just not much to clean up every week, whereas the gravel seemed to trap and hide away bits of food and such.
As for how much, my LFS guy said his rule of thumb was a pound per gallon. I bought three 20-lb bags and used two for my 50-gal. In terms of shock simply because of the new substrate - when my cories were moved from the QT (which has a shallow layer of gravel) to the new tank, they were just this side of jubilant.
One thing to watch out for as you're shopping for sand is that, if you're shopping at a pet/fish store, the sand is going to be focused at the marine aquarium market. As such, you will find sand mixes that contain shell, the calcium carbonate from which will have an impact on both your water hardness and pH. Stay away from anything that indicates that it has shell, or "calcium" or will help with water buffering, etc. Silica sand is inert in that regard.
Hope that's helpful...
Jen
. Keep as much of the original water and keep the filter alive if possible.
