Hi
Its caused by the carbonate hardness of the water. I used to use hard tapwater and needed a vast amount of nitric acid to bring the ph down. It was a nightmare as i`d keep adding acid and nothing happened until the buffering capacity was used up..at which point the ph would plummet to about 4 :)
I`d recommend using RO water or rainwater if pollution isn`t an issue, it`ll save you a small fortune on ph down.

Thanks for the reply guys! Not quite what I wanted to hear, but kind of what I expected. I guess I'll start looking for an RO unit.

Ok. I have a new question. I bought an RO unit. And it gives a PPM of 0 and a pH of 4.8. This is great! Except I'm wondering why when I add pH down it has no effect? I'm just playing around with the pure water right now and this didn't make sense to me. Is it because I haven't added nutrients yet and there are no salts to work off of or something? I'm using a hydroponic ph down. The pH up worked well and shot up the pH right away, and I overshot where I was aiming, then added pH down and it won't even move a .1?

Water chemistry sure is more confusing than I originally thought!

Something's wrong? The ph of RO water should be between 6.5 and 7.0, depending on the unpurified water.
1. What kind of container are you using to hold your purified water? Is it clean? Check your water in a clean glass to double check.
2. Have you calibrated your ph meter lately?

Urban has some good points. Definitely check out the pH meter.

Most pH lower products are something like 80% phosphoric acid and have pH of ~1. Try adding some lemon juice, which is a pH of around ~2 and see if that causes a noticeable change. If the lemon juice causes a shift but your pH lower still does nothing then it begs the question what's in the pH lower bottle.

I wrote a primer for hydroponic gardeners relating to pH levels and water chemistry. It's not so much about the problems you are experiencing specifically and it sounds like you already have most the background knowledge, but if you are interested you can find it here: http://www.bilberrybrian.com/Joomla/index.php?option=com_content&view=article&id=64:hydro-ph-levels&catid=42:hydroponic-gardening&Itemid=64

Another thing to try is taking ground up chalk and adding a few drops of your pH lower and seeing if it forms bubbles. If not that would mean the stuff you have in the bottle isn't really that acidic.

It's more likely the pH meter is out of order than a bad bottle of pH lower that slipped out of the bottling factory. Worth checking out though. Let us know.

Hey, so I was going to post my own thread on this, I am not hijacking, but my thread really is an answer to his question. I have hard water, 7.5ish, and like was said, mine floated back up all the time, I had to constantly monitor it and use ph down.

Then I was at the aquarium store getting supplies and I noticed this stuff called "tap water conditioner" and it was advertised as a dechlorinator, breaker of the chloramine bond, and remover of heavy metals. So I bought some, it was four bucks for 118 ml and I use 3.75 for 55 gl of tap water. It LOCKS the ph at 6.3 and obviously lowers it to get it there, I don't use ph down until after I use it. It's made a big diff for me, for my soil plants I don't have to worry anymore about lockout, and hydro its much easier for me to get to what i need.

I do agree RO is better for lots of reasons, but more costly.

So that is my answer to his question, but this stuff doesn't say on the bottle whats in it or if its safe for humans, so my thread was going to ask whether anyone knows if its safe and has used it before in hydro with success?

Hope this helps