You could always use sand (if you haven't already). I went to wallmart and got three 20 pound bags of blue play sand for a total of like six bucks. it looks great in my 55 gallon tank and was way more budget friendly than putting new rocks in there. The only thing i worried about was the filter intake sucking it up (hang on back filter) but i got this stuff from were i work that allows water to pass through it but blocks the sand. I think its called dell net or something like that. I just wrapped it around the bottom were the water comes through.