look into some 1x3 oak, cherry or maple boards
using a mitre box and saw, you cut a board the proper dimensions to make a rectangular box the size of your tank
then using more of the same board, make 4 legs in an L-Shape that the box sits on... use small scrap pieces to attach the legs from the inside of the leg to the inside of the box
next, you can keep using 1x3 or go with something smaller like 1x1 square dowels ...
build a smaller box that fits "inside" the legs, position it about 3-4 inches from the "bottom" of the legs. you could put your 5 gallon on this shelf, with the 10 gallon up above
finish with whatever stain meets your tastes, or paint, or whatever
should cost between $20 and $60 depending on the boards you choose (furinature grade oak boards get pricey), the only fasterns you'll need are some 8 penny trim nails and some small screws for attaching the legs' tabs.
1x3 should work fine for any size tank up to 20 gallons ... after that you'll want to move into 1x6 or 2x4 for the extra weight