What type of biofiltration do you have now? Is there an live rock or live sand in the tank? In addition to what biotoxin mentioned, most filters that use sponges or "bio balls" as a bio-filtration media will leave the tank with high nitrates. The bacteria that breakdown ammonia and nitrite flourish on the sponges and balls. However, the bacteria that breakdown nitrates into nitrogen gas need a low oxygen enviroment to grow. Deep sand or highly porous LR
can provide lower oxygen areas for these bacteria to grow. Adding LR
to your 125 gallon would be pretty costly though. If you already have enough LR
then you may want to remove any other biofiltration media and let the rock and sand do the work. Are you running a skimmer? A skimmer may also help control the nitrate problem.