I've kept both a Topaz and South American puffers in community tanks, at one point all together. Although just because it worked for me doesn't mean it will for every body. My Topaz was fine with all of my fish excluding noens that he decided to eat all of over night one night :(
I was told...
I've always thought that fish are a personal thing and as such it should be your choice and your choice only. Obviously you need to factor your water conditions into the equation so its probably best to wotk out what fish are suited to your conditions and then decide from there. Me, i love...
My honest take...... its scary as hell. Alot of very quiet giants are being awoken that should have been left to sleep. This amount of unrest is not good for the world and could lead to a lot of serious problems for every one :(
At present i use a Tetra CO2 pressurised can with one measure a day plus a does of Seachem Flourish twice weekly. Something tells me thats not enough though, maybe i'm wrong.
Its a 125l tank if that makes any difference with a mild planting.
Builders sand frequently has chemicals mixed into it that help it to bond when mixed up for concrete and what not. Personnaly i would stay clear but its up to you.