Is it normal to knock your pH down every two days or so even when no plants are in the system?

I have two floating raft tubs with 9gallons of tap water in each that started out at a pH of 7.8 and ppm of 200. I have a 12" long bubbler in each tub also.

Over the course of the week i have added Hard Water nutrient and 1 Tbspn of pH down to acheive a pH of 6 and ppm of 900.

The next day my pH was up to 6.4 today the pH is up to 6.8. My ppm has stayed nice and stable at 900.

This is with no plants in the water.

Should my pH rise like this? What would cause the pH to rise?

I cleaned out the tubs with dishsoap before using them and the foam i am using is the white foam you get at the hardware store. I am using (cleaned) clay balls for the planting media.

If this is not common i think i am going to toss out the water and use some bottled water from the grocery store or get a filter that i can put on my kitchen sink.

Any recomendations on filters?

We have a britta filtered pitcher that i took a measurment on and it was 7pH.

If the pH rises again i am going to tear everything down and wash it.

I did not wash the baskets the clay is in and i did not wash the white styrofoam boards that are floating. So maybe there is something on them that is causing the pH to rise.

I think in my book it says to use hydrogen peroxide to wash out the tubs. Is there any other cleaning solution i should use?

In your testing of the water supply. Here are some other things that might be useful to know. When the water first comes out of the tap, it may have a different pH than it will after being aerated. This is because there is likely disolved CO2 in the water which will act as a weak acid. For instance, my well water when it comes out the tap will read a pH of close to 7. After letting it sit in a container with an air stone running in it, it will read a pH of 8. WOW that is quite a difference. I had not originally known that I should bubble the water before testing the pH and I was so confused about how my pH kept climbing when the tap water tested as 7.

So, I don't think the pH problem is one of cleaning.

I actually took to using rain water for my Hydroponics. It also tended to test with a pH of 7 but it didn't have the amounts of calcium in it that cause the to keep trying to climb. That way I was able to avoid daily pH adjustments.

Another trick that some might try is to use a naturally acidic media in part of the system. Perhaps something like coco coir or peat. I actually used wood chip mulch in my outdoor system to grow cucumbers that like a rather low pH.

Now I've been converted over to Aquaponics where the bacteria doing the ammonia-nitrite-nitrate conversion from the fish waste tend to cause the pH to drop over time so the challenge is opposite.

It does appear my pH is becoming more stable. The last 3 days the pH has stayed at 6.5 for tub1 and 6.9 for tub2. I have also started taking my measurments at the same time of day. I read pH will change during the course of the day.

I have about 5 more days before the lettuce sprouts are ready to put in the system so if the numbers staty stable i will use it.

I have heard of aquaponics and it is interesting, but at this time i want to keep things simple. I was at the pet store a week ago and did check out some of the neat aquariums they had. I could probably get into that hobby.

Yes, pH will change through the day. Over night the plants will strip disolved O2 from the nutrients and there will be more CO2 in the water, causing the pH to be lower first thing in the morning.

Here – in Africa – rain is at a premium, and many things are not readily available, so . . . we DIY a lot.

If tapwater is too hard and/or the pH too high we manage to deal with that with simple means.
Hardness is a result of the calcium and magnesium content of the water. It can be measured using a hardness tester bought at an aquarium shop. The test provides two measures:
GH (for General Hardness) is a measure for the total of calcium and magnesium content given in mg calcium oxide (CaO) per 100mL of water.
KH (for Karbonate Hardness) is a measure for the calcium and magnesium bicarbonate content. Also given in mg calcium oxide (CaO) per 100mL of water.
Naturally, the KH is part of the GH, and is always smaller, unless there’s a foreign bicarbonate in solution.
Bicarbonates are strong pH buffers which resist lowering of the pH.
The best way is to store the water under an aerator for a few days, and bring the pH down using a measured quantity of 5% white spirit vinegar. This will turn the bicarbonate ions into water and CO2 gas which will be driven off by the aerator. What’s left is calcium and magnesium acetate.
Quantity: 0.4mL vinegar per Litre of water, per degree KH hardness.