This may be your problem! I looked up the cat litter you used as substrate and it appears to have added chemicals to control odor:
Premium Clay Cat Litter
For pet owners that demands the best, there's Special Kitty Premium Clay Cat Litter. Its odor control system traps odors while releasing a pleasant fragrance that keeps your cat's litter box smelling fresh and clean.
I'm betting that is what is making your parameters wonky. I've never been huge on cat litter as a substrate simply due to not knowing what kind of crap they put in it other than clay. If you are looking for a super cheap substrate that works, look for Turface ProLeague. Its a fired clay substrate but it's totally inert and has a high CEC so it soaks up ferts and holds them well. I used it on my 55 gallon and had great plant growth when combining it with Osmocote.
Pro League® Calcined Clay | Turface Athletics
Find a Distributor | Turface Athletics