Were you planning on switching to normal output lighting? Your rock most likely will not retain its color if you switch to low lighting, but without corals, your water changes would keep your calcium and alkalinity replenished. I'm not sure about algae....if you maintain your tank, then it probably won't be an issue. Algae is a sign of too many nutrients, so you would want to limit what's left in the tank (i.e. overfeeding, not doing maintenance, etc.). You don't have to be as strict with water quality with a fish only, but you still want to maintain good water quality to keep your fish healthy.
Maybe the best thing would be to determine why you want to switch to fish only? If we know the reasons, then maybe we can help you decide what the easiest route would be to switch the tank over.
Hope this helps.