Blackwater (low pH, low KH) tank cycling

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Fish Crazy
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Dec 10, 2023
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Pacific Northwest, USA
ATTENTION: See third post for updated thread topic. My concerns listed below are no longer relevant, but they did lead me to the current thread topic of cycling a tank with low pH and KH, so I decided against making a completely new thread.


I set up my 25gal yesterday. The tank is less than 24 hours old at this point. I just tested the water for the hell of it because I was curious. I wasn't expecting to see anything notable, but it turns out there's some ammonia hanging out in there. I'm curious as to why, because I did not directly add any. I'm not following the full fishless cycle process since I do have a 5.5gal tank that's cycled already. I put some gravel from that tank into the 25gal and in a few days, I'll put the old filter cartridge in the 25gal as well, once the new cartridge in the 5.5gal is seeded. The only other things I added to the water were API's water conditioner, Seachem Stability (expired), and API's quick start (not expired). I'm not expecting the bottled bacteria to really do much, but I figured it couldn't hurt. Could those be causing the ammonia? Seems unlikely to me, but I suppose it's a possibility.

There's also some frogbit in the tank that I put in yesterday. It had been sitting in a bowl of water for a couple days while the tank was being set up. Maybe that was enough time for things to start decaying? But I did rinse each plant in clean water and pruned off dying leaves before placing it into the tank. Some plants are slightly yellowed today, so I'm wondering if the ammonia is damaging them. Edit: I'm also noticing they're dropping a lot of roots

The things I think are the most likely causes of the ammonia are either detritus that came with the gravel from the 5.5gal that's now decaying, or the wood that's in the tank. A few pieces still have thin layers of bark (edit: more specifically, three thin branches have some very thin papery bark). None of the wood appears to be in any state of decay though. I'd be surprised if it started breaking down in less than 24 hours.

Should I do anything about this? I am a little worried about the frogbit, as I do want it healthy and growing to help the cycle along. Should I dose with Prime to at least detoxify it? Do a water change? The water's also ever so slightly cloudy, although I'm running a sponge filter rated for 40 gallons.

Full parameters, using API's liquid test kits:
Ammonia - 1.0 ppm
Nitrite - 0 ppm
Nitrate - 0 ppm (maybe like 1-2ppm, the color isn't perfectly yellow)
pH - 6.8
GH - 18 ppm
KH - 35 ppm
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Did some more asking around and I'm no longer worried about the ammonia's effects on the frogbit. Seems like 1.0 ppm wouldn't be enough to damage the plants and that their condition is more likely due to damage and/or temperature shock during transport.

Still curious about the source of ammonia and the slightly cloudy water though.

Edit: reading other posts on here, especially the ones from the lovely Essjay, have assuaged my cloudy water concerns. I rinsed the substrate well and the water was clear upon first fill, so it must be bacteria. Somehow it completely did not occur to me that simply dechlorinating water is what allows the bacteria to bloom. Makes perfect sense. I'll be patient and wait for it to die out :)
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Changed the title of this thread because I've decided it actually doesn't matter where the mystery ammonia came from. It'll get eaten up by the plants and whatever bacteria is in the gravel. And anyway, I've decided to just proceed with the proper full fishless cycling method.

The thing I'd like to know is how to balance the needs of the cycling process and this tank's eventual purpose. This tank will be an Amazon blackwater tank with species that rely on very soft water (apisto, hatchets, otos...). My tap water is naturally very soft (GH/KH are both ~35ppm), which is great for the biotope. And, when I add botanicals, the pH could easily drop to about 6.5. Currently, without botanicals, the tank water is 6.8. However, I know that growing BB requires a bit more KH and a slightly higher pH.

Would I have to raise the KH in order to cycle the tank? If so, could I raise it temporarily during the cycling process and then bring it back down, or would that kill off the bacteria colony? Same question for pH. Could I complete the cycling process with my current parameters, and if so, would it just take longer?
One thing to note is that I'm using gravel and filter media from an existing tank to speed things along. Obviously, that tank's water is also very soft. Is it possible that the bacteria colony in there is already adapted slightly to low KH?

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