Fish tank setup update

Old tank:

• 10 ppm nitrate
• 0.25 ppm ammonium
• 0.5 ppm phosphate
• pH 7.6

New tank:

• 20 ppm nitrate
• 0.5 ppm ammonium
• 0.5 ppm phosphate
• pH 7.6

The ammonium in the old tank is a bit troubling; perhaps the nitrifying bacteria population isn't big enough without that giant floating plant mass acting as a nitrogen sink. Or maybe uprooting the transplanted crypts released underground ammonia? That seems unlikely. Anyways, the animals (nerite snail, shrimp, oto, 2 panda cories, 5 harlequin rasboras) seem to be doing fine.

Initially I thought the ammonium and nitrate in the new tank must have come from the soil, but ammonium levels have been around 0.5 ppm for a week now. I hope it's not steadily leaching from the soil (through 2" of sand); I think it's more likely to be decaying algae on the rock and the log (leftover algae from the old tank, dessicating for almost a year). The new grass seems to be doing all right; some of it has withered away but that just means the remaining leaves are thriving, right? Crypts look all right, except the leaves that I broke while transplanting it - those have died. Stem plants that I took from the old tank are all doing very well. The ones I'd taken from the floating plant mass don't look so good. They started out with smaller leaves, and now many of them have yellowed (probably dying) shoot apical meristems.

Today I did a 50% water change of the new tank and a 20% change of the old tank.