Possibly never, that tank is bare minimum for oscars, but the 125 is a much better option for them long term. Why wait? If the tank is availabe upgrade them now. In either tank the two most important things are the quality of the food and water. I only feed and recommend NLS Thera+A. For water quality you need to get in the habit now of doing large weekly water changes, don't let the nitrate get over 20ppm before a water change. Oscars are extremely prone to HLLE and in oscars it is almost always caused be low quality food and/or water.