have you vacuumed the substrate and cleaned out the filter pads lately? the low ph can also indicate there is a lot of waste in the tank.
High ph? its 6? I would get the ph up. The bacteria that eat ammonia start being less effective at about 5.5. True that the higher the ph the more ammonia hurts the fish, but 6 is too low for oscars. Have you tested for hardness as well? The filter is new but the tank is old am i right? With low ph and high nitrates I would say that the kh buffer is all used up and you it could use a bump. Here is a good read: http://www.bestfish.com/oldtank.html If your KH is low you can raise it with baking soda.Hoggy Boss said:I don't think it's done cycling.( the 405) Slow down on the feeding and continue to do small water changes to help reduce nitrates. I wouldn't mess around with the salt, because high ph will cause the nitrates to become toxic at that level.