The tank is only 20 litre so I guess that about 5 gal and I will have a beta in it. Pool filter sand was my first choice, I went to the pool shop closet to me and a 20kg bag was $20 for 1mm grain sized pfs. I've also checked out Bunnings (a large Australian hardware store) and the had river sand for $8 for 20kg and coarse sand for $8 for 20kg. Both seemed suitable and very well priced but my only problem is i don't have a car to transport such a large bag, I try to ride my bike places. And on top of that I only need a few handfuls



