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Get some of that gravel and pop it in a bucket or container. Put some conditioned tap water in and measure the initial parameters (GH, KH, pH, TDS). Give it a week or two and then measure again. Do the same with the rock. Easiest surefire way to see what the problem is. If it’s neither, then you’ve got some sleuthing to do.

Thank you both so much for all your help (and apologies for taking a while to get back to this site - had some health stuff come up but all is well now!) I got my tank light in (hygger 18W) and it looks SO much better. It seems that my tank is nearly cycled. Ammonia drops from 4ppm to 0ppm in close to 12 hours, but I'm waiting for the 2nd set of nitrifying bacteria to catch up. Nitrites haven't yet dropped below 2ppm. Nitrates are around 80ppm. My KH dropped to 0 a few days ago, and soon after that the PH started swinging, so I did a ~75% water change using remineralized RO a few days ago. KH and PH are back and stable, plants and algae are growing. Everything seems good and I can't wait for those nitrites to finally drop off so that I can add shrimp! I haven't done any topping off yet, but noted - if I do, I will use pure RO water to avoid GH creep. I'd like to add floating plants soon (especially interested in red root floaters.) Do those of you who use floating plants leave the lid on or off your tanks?

@Crabby, good to see you still poppin' your head in here occasionally. Give us an update on what's new with your tanks (maybe in a separate post).