HM is a weed, it sucks out a lot of NO3 and the genus does well/better in richer water column ferts. This is very true for Mic umbrosum and this species.

If not NO3, then CO2.
You are not adding much KNO3, about 1/2 what many may add for this tank size.
You have a lot of light for this tank, 4x39w T5's.......that is a lot of light.

So high CO2 and NO3 demands. Even with ADA As etc, you'll still need more KNO3.
Many folks trying to run leaner systems often have this same trouble, especially if they slack off a little on the dosing routine, that's the risk you take with low steady concentration rates etc.

Add more, it does no harm, also, switch to adding CO2 in the day only.
You can add more with less fish stress and more for the plants.


Regards,
Tom Barr