No way to find out the kH, doesn't mention it on my water companies site. Can't be too bad, the other tanks are fine.
Thanks ricbea, glad you're enjoying it.
So day 13 now, haven't added any ammonia since day 1 and it has definitely been doing something. Haven't had to add any ammonia as my substrate is giving off plenty. I guess I'm just gonna wait it out for now. The duckweed was out of control and formed a thick mat on top of the water surface. Within a few days it outgrew the duckweed jail and was escaping under the sides, blocking the light from plants below. I've filled almost half a bucket with the stuff and it seems to be all gone. Ammonia and nitrite are as high as my kit reads by the looks of things. I've also ordered some water sprite as nick suggested, hopefully it'll act as an ammonia sponge and shade for the fish while also being easier to control than duckweed.
The plants are all doing well, the marble queen Echinodorus is huge now.

The mayaca is gaining in height every day as well.
I'm a bit concerned about the Vallis, it's melting off and many of its leaves are dead or dying. Is this just because of the stress of planting and shipping? Should I do anything or just let it rest and recover?
In the other tanks, I've found a pellet brand that stopped my axolotls floating. All 3 have had no issues with staying at the bottom of the tank for over a week. I've upped their feeding and they're all getting 3 pellets each a day now. They seem to enjoy being able to remain submerged and explore all parts of the tank throughout the day.
Along with the water sprite I've ordered some more tetras for the 90 liter. The flame and rummy schools are quite reclusive I assume due to being in a rather small school. The plan is that when the new tetras arrive they will feel more comfortable and be more outgoing.