Hi... I set up my new tank around 4-5 weeks ago. I think its finally cycled. The problem is that my PH is pretty low. Its about 6.5.

My ammon is finally at 0
Nitrite is 0
Nitrate is around 40 (doing a water change tonight)

I have been doing all my water changes with R.O water and adding R/O right to the RO water when I add it.

Why is my PH so low?

PS... In my tank I have a pleco, a pictus cat, 2 clown loaches and about 10 tiny danios. I will be adding a tigar oscar soon.

Firstly, if you are keeping South American fish, a pH of 6.5 is nothing to worry about If you keep fish from around the world than a pH of 7.0 is advisable because it is neither acidic, or alkaline, it is known as neutral. Okay, pH can be influenced by many factors, one factor is fish waste. Fish waste lowers the pH. What I suggest you do is test the pH that comes out of your tap. Water changes are what controls pH in many cases. Let's say you've got a pH of 7.5 coming out of your tap. Regular water water will help stop it getting too high, or too low. Obviously if you have an overstocked tank, you may find your pH dropping very quickly. This is why keeping the fish in the correct environment is so important.

Ok... but would reverse osmosis water have anything to do with my PH being so low? I use reverse osmosis water, not tap.

I have to confess that I have never used any type of reverse osmosis so I can't give you any advice from experience. However, I believe that using reverse osmosis can result in low pH so this could be why your pH is low. Having said this, the pH may still be low without the reserves osmosis. However, take on board what I've already said, your pH is perfect for Oscars and any South American fish. If you plan on keeping fish from other parts of the world, I would check what sort of water conditions they require.

Don't worry about your pH, your Oscars will be right at home with pH of 6.5

A RO unit will remove most impurities and that is why it will lower PH. I use a RO unit for my discus tank and the PH comes out real low. I then mix it 3 RO to 1 of tap water to get the water about right for my discus. Using pure RO water is not a good idea, apparently (and i am not that sure on this) but fish need some trace minerals in water, also with pure RO you can end up with PH crash. I would advise mixing your RO water with tap water. This will add the trace minerals and also raise your PH. As OFL a PH of 6.5 is ok with oscars but i would add the tap water to stabalise the water. I know very little about all this stuff about PH crashing and using pure RO water however i can tell you that i was advised that to use pure RO water is not a good idea