They don't make 125 watt MH
's. Do you mean 175 watts? And the light by itself will not cause algae problems, it's a combination of lighting and excess nutrients.
's usually cover about a 2'x2' area with light. 30 and 37 gallon tanks are 3' long so then ends of the tank will be dim.
It is possible you'll have to cycle the tank again. Are the tanks you are looking at currently setup and running with lr
? Also make sure to ask the seller if they have ever treated the tank with copper medications.