Concentrate on your ammonia, nitrites and nitrates. I do hope you will upgrade in the future because your tank is already showing signs that the nitrates may not be kept under control without doing plenty of water changes.goosej13;1323083; said:I have a 40 gal tank with 5 RBP's in them at about 3- 4 inches each... and yes they are really piranhas... haha.. I just did a 30 or so % water change... see how that helps... what kind of water hardness should i have?? or does that matter?