Cycling and PH unstable

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TLS_70

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I've been cycling a new 100g and thought I was there. After 3.5 weeks, my numbers were coming back perfect 7.6 Ph, 0-Ammonia, 0-Nitrites, 80-Nitrates. I was following the same protocol I used to cycle my 55 - fishless cycle, using FluVal Bacteria Booster and Fritz Ammonia and the timeframe was about the same. My 55g has been rock solid since (~6 months). However, about 3 days after I thought my 100g was cycled (keep feeding it ammonia and testing to make sure everything stayed at 0), Ammonia spiked. The day before we had a power outage for ~15 hrs and the filter was not running, so I assumed I lost the cycle and needed to start over. Since my Nitrates were so high, I did a 50% water change and started over. This time around, I did not use bacteria booster, but instead pulled some rocks from my 55g to help jump start it. I'm about 2 weeks in and I'm noticing my Ph is bouncing (7.8 one day, down to 6.8). I added Seachem Alkaline buffer (4 teaspoons) and it went back up to 7.8, but the next day dropped back to 6.8. I just added more Alkaline buffer and will check again tomorrow. My KH is ~155ppm and my GH is ~100ppm. My tap water is normally around 7.6, so not sure what is going on. I'm keeping Mbunas in my 55g and will be moving them into the 100g. The only difference in the two is that 55g has Texas holy rock and the 100g has slate. Not sure what to do to stabilize the PH. Any advise here?
 
From what I am reading it seems like you want a more Alkaline pH in your tank. One difference is the rock. Slate is a metamorphic rock which should be nearly chemically inert although If the slate has many pyrites it could also be pushing the pH down, pyrites are sometimes common in slate. The Texas Holey Rock is clearly a buffer releasing carbonates into the water driving the pH up and it is marketed as such, so the difference in the rocks is a big difference to my way of thinking.
 
Have you tested your tap water after leaving it to stand for 24 hours?
 
From what I am reading it seems like you want a more Alkaline pH in your tank. One difference is the rock. Slate is a metamorphic rock which should be nearly chemically inert although If the slate has many pyrites it could also be pushing the pH down, pyrites are sometimes common in slate. The Texas Holey Rock is clearly a buffer releasing carbonates into the water driving the pH up and it is marketed as such, so the difference in the rocks is a big difference to my way of thinking.
Thanks for the response. This would explain why I have no issues in my 55g, which is all Texas holey rock. I was pushing for higher Ph with Mbunas, but have also read to not stress the Ph too much, just wasn't sure what would cause it to bounce so much.
 
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Very interesting about the pyrite in slate, I wasnt aware of that! Had a read and apparently the pyrite reacts with water to produce sulphuric acid.

@TLS_70 what substrate do you have in the 100g?
 
Just to be clear Pyrite in slate is typically localized to various areas. Pyrite, and irons in general, occur in many sedimentary and metamorphic rocks from different locations. It is always a good idea to examine the rocks you use, both for carbonates and mineralization. I haven't seen pyrite in any rocks for sale in aquarium stores, it seems most suppliers are careful in that regard.
 
PH will fluctuate during a cycle, perfectly normal.

Trying to manipulate ph levels during a cycle will only complicate matters.

Once the cycle is done, ph will stabilize, if other factors (such as the rocks discussed) do not influence the ph levels.
 

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