Sand is fine as long as it is not the sharper quartz or black sand. Felis gave you many good feeding options and will find more in here: http://www.frogforum.net/care-sheet-...warf-frog.html. Although Sandy strongly advocates live feeding; you can also feed same foods in frozen versions if live is not available locally. Do not feed freeze dried foods to frogs! Some owners do feed their aquatic foods pellets like Reptomin. If using those, will need to crush and maybe pre-soak them so it fits in frogs mouths.

Myself recommend keep the frogs to themselves. When getting companions, you might want to consider the possibility of introducing pathogens with them and quarantine the new ones in a separate tank for a couple weeks to insure they are OK.

If your filter is working OK with frog do keep it. It does a great job harboring beneficial bacteria that reduce ammonia and nitrites in tank. As long as filter flow allows frogs to reach surface and pause there to breath; it will be OK. Good luck !