I do weekly without exception and if I see a little cyno, algae or anything indicating nutrients have crept up a middle of the week water change isn't out of the question (Just did one today matter of fact, only about 2.5 gallons). having only a 30g water changes are really no big deal and I only do 5 gallons weekly. I could probably go longer but with this routine I dont even have to run a magfloat across the glass between changes if I dont want to. everything stays pretty clean. On the biocube 14 I do 2.5-5gallon weekly changes (its fairly new, skimmerless and going thru the new tank progressions so no amount of water changes keep it looking good without elbow grease). the smaller the tank the more important staying on top of water changes are IMO. when I had a 100g I only did 15g every 2weeks and it was ram packed with fish and coral, fed 2-3x daily and did fine. I could never do that with the current tanks without issues.