With your current filtration changing 30% every 3 weeks is not enough. You should be doing 50% weekly or 25% twice a week alonge with cleanning/vacuming the substrate. Ideally you want your nitrates as low as possibile and 20 is a little on the high side for Oscars. The low level stress of high nitrates on oscars make them suseptable to illness (hex) etc.
This looks like more than a srape to me. It could be the early stages of Columnaris. What the poster above discribed is a classic example of how quickly it can kill left untreated. Unplug the heaters to dop the temp into the mid to low 70's. Add 1 tablespoon of salt for each 5 gallons of water and treat with API triple slufa. In a 240 gallon tank this isn't going to be cheap but unfortunatly the whole tank will need to be treated. Do a large water change (50% or more) and vacume the substrate before starting treatment. You can use slightly cooler water than normal to help lower the temp but don't go more than 10 degrees below the current temp when doing this. When treating with 3x sulfa it is important to follow the directions on the package exactly. If you are running carbon on the tank make sure to remove it befor starting treatment. Keep us posted.
This looks like more than a srape to me. It could be the early stages of Columnaris. What the poster above discribed is a classic example of how quickly it can kill left untreated. Unplug the heaters to dop the temp into the mid to low 70's. Add 1 tablespoon of salt for each 5 gallons of water and treat with API triple slufa. In a 240 gallon tank this isn't going to be cheap but unfortunatly the whole tank will need to be treated. Do a large water change (50% or more) and vacume the substrate before starting treatment. You can use slightly cooler water than normal to help lower the temp but don't go more than 10 degrees below the current temp when doing this. When treating with 3x sulfa it is important to follow the directions on the package exactly. If you are running carbon on the tank make sure to remove it befor starting treatment. Keep us posted.
