GH PH DOWN is made from phosphoric acid, which makes your reservoir pump and hoses slimy, especially during flowering. When the phosphoric acid hits the P saturated solution, a plume of phosphorus hits the water and the solution cant hold it all, some of it changes form to a solid and it becomes slime that sticks to everything it touches. I use sulfuric acid and I get no slime at all, all I have to do is rinse out my system once a week, almost no scrubbing at all.
You can get sulfuric acid at any auto parts store, just ask for battery acid. Cost is anywhere from $3.99 - $5.99 per liter, very cheap. You have to ask for it, they keep it behind the counter. Wear rubber gloves and goggles while handling it. For weaker strength, use 1/3 of a cup per gallon. For normal strength use about a half cup per gallon. Make sure you use RO water or distilled water.
You dont want the chlorine from your tap water interacting with the sulfuric acid. A $3.99 container of battery acid will make about 8 or 10 gallons of PH Down.
For PH UP use 100% pure lye. One level tablespoon makes a gallon, which lasts years. Lifetime supply for under 5$.
Read the ingredients on the lye to make sure it is PURE sodium hydroxide or pure Lye in crystal form. To make the PH UP, fill a gallon container 3/4 with RO or distilled water and drop one tablespoon of lye crystals in, then top off with the rest of the water. Wear rubber gloves and goggles. I will never ever have to pay 25$ per gallon for UP or DOWN again!!!
Get it online cheap:
http://www.amazon.com/Sodium-Hydroxide-Caustic-Cleaner-Making/dp/B0032LVYXQ/ref=pd_sxp_grid_i_1_0
or get it at the hardware store:
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Just make sure it is pure lye or pure sodium hydroxide
Hope this helps.