High pH and GH due to substrate...questions?
Been a long time. Thought I'd run a scenario past you...
My 100 gal high tech planted tank was established in April of 2010. I have run pressurized CO2 since the beginning, having mixed success with several species. Within the past year, I changed over my moderate lighting (4x65w CFLs) to LEDS and I immediately saw an increase in growth, especially in my reddish colored plants.
I established this tank with the following mixed substrate:
EcoComplete (2 bags)
Seachem Fluorite (3 bags)
Pool filter sand (partial bag)
Zeosand (partial bag)
Hartz ph5 kitty litter (7 small "dollar" bags from dollar store)
Hyponex potting soil (partial bag)
All of this may, or may not have been a mistake. The partial bags were very much equal parts to everything else, so it was a very uniform mix.
My tap water (not changing much over the years) is pH 8.0, with 8kH and 11GH. TDS is approximately 160 ppm.
Running a drop checker and trying to keep my CO2 around 15 to 20ppm, I was able to keep the pH around 6.6 to 6.8. When my CO2 would run out, the pH would raise to 8.3.
Last year, once I added reef aquaria to my skill set, I invested in an RO/DI unit and I immediately started using 0 TDS water in my 100 gal tank. Over time, while using the same levels of CO2, my pH began to rise to 7.4/7.5. I was changing out water often at this point, and my TDS meter showed 300+ ppm after the first several water changes down to a current 250 ppm. Today, when measuring GH, I stopped putting drops into the API test tube at around 40 drops without it changing color.
Yesterday, when my CO2 ran out, my pH jumped up to 8.8.
I pulled out some substrate today and put RO/DI water in it...the pH measured 8.4.
It's obvious that the RO/DI water does not buffer like the tap water did, but I never used RO Right (actually I have Barr's Mix) because of the always high TDS even after the water changes. I was hoping and waiting for my pH and water hardness to naturally go down before I saw the need for adding the buffer.
I know that EcoComplete had a questionable reputation with regard to pH at that time, but you'd think it would wear off by now? Anything else that could explain it? Shouldn't the Kitty Litter be driving down the pH?
Anyway, I will likely redo the tank with a better substrate, like with dirt or aquasoil, but I thought I would ask here what you folks think is the dynamic I am seeing? I'd like to be able to grow some good plants!
I do not blame you if you TL;DR. My apologies!