People usually suggest between 1-2 lbs rock per gallon but that rule is loose. I probably have less then that. As long as you have enough surface area to keep a large population of bacteria you should be fine. I would probably keep it closer to .5 -1 lb per gallon for a fish only since a lot of those fish need more swimming space. You can also put in a little deeper sand bed to provide more surface area or put addition rock or rubble down in your sump.