http://delloyd.50megs.com/moreinfo/buffers2.html talks about using potassium dihydrogen phosphate ( Monopotassium Phosphate) as a buffer amd how to make it.

Here is a link that might be useful: http://delloyd.50megs.com/moreinfo/buffers2.html

I'm not looking for something so technical, like I said I'm just an amateur. Basically the type of answer i'm looking for is along the lines of, should I dump a teaspoon of the stuff into 5 gallons of water, once I've brought my Ph down to desired range using phosphoric acid.
Real simple insturctions on how to use this stuff to stabilize the Ph, using rough measurements is what i'm looking for. Thanks for those links though I will check them out to get a better understanding.

It's not quite that simple. first you need to see where you water is sitting. simply set a known amount (maybe 3L) out for several days then check its pH. if the pH is already higher than your desired range, you'll need to add some kind of acidic buffer to bring it to the required range. let it rest to stableize, then check the pH again. if you take it too low (maybe 4) then raise it to the desired point (5.6 though I would aim more towards 6.0) with the potassium phosphate gradually allowing it to rest each time and checking the pH before adjusting again you should come up with a decent buffered solution. just remember to record all relevent data each time, so when you try to recreate the solution again, you know where to start.
This guys site does a decent job of explaing what has to be done to buffer a solution. he tries to write it in layman's terms, but you'll still have to read slowly and absorb it.

That is a great link grizzman just the kind of info I'm looking for. If any more pop up please post them as well. Many thanks.

@ njayjay

Monopotassium phosphate is actually not considered as an additive. It's a basic key component in almost any hydroponic nutrient solution. It's the classic component that provides all the phosphorus needed (and some of the Potassium) in the formula. As it is a acid component it is supposed to lower the PH enough to be adequate. Together with other components it buffers the PH. Hence the content of Monopotassium phosphate in any formula is quite determinant in the first place.

Also, because it provides the exact wanted quantity phosphorus to the formula, Monopotassium phosphate is used at a precise amount. Adding more Monopotassium phosphates will not only bring excessive P, but leading to some antagonism which may cause problems with micro nutrients and calcium uptake.

Are you sure you are having REAL RO-water, or is it post-treated (re-mineralized) or softened afterwards. Normally RO water does not have such Ph-fluctuations. This is more typical for any kind of hard or re-mineralize (softened) water as it has a high alkaline PH buffer due to the minerals (natural or added) that neutralize any acids. RO water is supposed to be "empty" and be ideal for lowering PH by simple add of balanced nutrients.

Why wanting to lower the PH down to 5.6 anyway?
Best is always to get the PH down to nearly 6 with the use of your nutrients that are supposed to bring your PH down and buffer it. Only do slight PH-corrections with any commonly used acid. It's much better to target a stable PH that is near the target than having a "ideal" fluctuating PH!
The ideal way is actually to bring it down to 5.9 and letting it climb slowly to 6 point something before correcting again. But if this is not possible to realize with standard means,....

If your water quality and your nutrients bring you way of the target and seem to have no buffer, either completely change water or nutrients. Pottering around with a bunch of substances will only cause more trouble.

PS: about the buffering technique described by Daniel Fernandez (link provided by grizzman).
I was firstly quite amazed about the citric acid/potassium carbonate PH-buffering method he proposed at his website. So I asked him (by email) what his latest results were. But unfortunately (was anyway to nice to be true), he had to tell me that - Quote of Daniel himself:

"This buffer has several problems, particularly the precipitation of calcium citrate from the nutrient solution. Truth be told, this buffer must be used very carefully and only in very moderate amounts to prevent this from happening."

Hence the description provided at the link given by grizzman, has to used with care as well or must be applied differently (more sparingly) than discribed!