pH unstable at end of fishless cycle

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Aquarium Advice Newbie
Nov 26, 2023

I am nearing the end of a long and challenging fishless cycle - about 2 months. Many weeks of troubleshooting, high nitrites needing many water changes, etc. Almost at 24 hour turnaround on 2ppm ammonia, 20 gallon freshwater tank (my first one!). 84. degrees for the cycling purposes. Planning to start with a few harlequin rasboras (6 at first then hopefully add 2 more), and then later on if all well some corys.
I have been fortunate to have lots of great advice online from some very helpful people in getting through this cycle, and thankful for any feedback on the following:
Ph has been funky and unstable. Tap water ph is 6.6 (after sitting about 24 hrs) , kh 2, gh 3.
Tank ph was at 8ish for a while, as per helpful recommendation to help establish the nitrite consuming bacteria bc it was lagging greatly. (used baking soda). When the nitrates finally started increasing, the ph went down to 6 and halted the cycle. I brought it back up to 7.4 with water changes, and it recently again tanked to 6. When the ph is 7.4, the kh is 3. The gh is 5 now bc I added replenish last week.
No live plants in tank, gravel substrate, artificial decorations, air stone on. Marina filter.
I am excited to be almost done with the cycle, but worried about how I will be able to maintain the ph when fish are here?
Thank you!!!
You want to try and keep the Ph above 6.0 to keep nitrification from stopping and since your tap water is 6.6, you should be good to go for fish species not requiring a higher Ph or higher GH. Nitrification will be slower in the 6.6 water so you need to add things slowly and test on a regular basis for nitrites to make sure you haven't overloaded the system. At 6.6 ( actually anything under 6.8), ammonia is converted to ammonium which is not toxic to fish unless it's really high so no real need to be testing ammonia as long as the Ph does not rise above 6.8.
If you wanted to naturally raise your KH and Ph, you can add some crushed coral rock or shells into either your substrate or into a bag and place in your filter. If the water starts to get too acidic, these will naturally start dissolving releasing the calcium carbonate in them to raise the PH and KH.
The main key is you want stability over chasing a number. There are a number of fish that like being in slightly acidic water so you have a lot to choose from HOWEVER, not everything likes acidic water so you need to do your research before getting new stock. 6.6 PH is just above the lower scale of acceptable Ph for many cory species so you need to find the ones that like it more acidic than alkaline. If you can keep your Ph about 7.0, that opens you to a number of species. (y)

Hope this helps. (y)
One thing to be aware of is you are dosing far more ammonia during a fishless cycle than would be the case with a tank full of fish. Cycling out this high ammonia depletes your KH and causes pH crashes much quicker than the lower amounts of ammonia your fish produce. When you are simply managing the bioload of fish, things will settle down somewhat and regular water changes are usually enough to replenish depleted KH and keep your pH stable.
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