Is a 30 gallon tank I have, big enough to convert into a sump.. For a 125 gallon tank? Deme mentions just fit right, under the stand & saves me from spending n find new demensions for heater & stuff.
X2. No Law or rules if it fits, it will work.A sump has no "right" size. The purpose of a sump is to add water volume, hide equipment from DT, aid in WC and to house a fuge if this is your choice of nutrient export.
In the case of a sump I would say the bigger the better! but its your choice depending on your plans. If 30 gallons is whats going to fit in your stand and house all the equipment you are going to add and you have no other option that is fine. Make sure tho you are set up right that your sump is not going to overflow in the case of a power outage.