I just bought a used 120 gallon fishtank and wanted to know what I could use to clean the inside of tank with out letting it hurt the water while I cycle and I will be letting it cycle for about a week with chemicals after I clean it.
+1 But you can also get a spray bottle from the dollar store and a jug of vinegar. Mix it with water in a 50/50 solution. Cheaper, and can be used as a general household cleaner, too.
If its a build up from marine coraline or salt deposits etc you could use a plastic card like a credit card etc hard work but it does'nt scratch the glass or harm the water