Are you using the Thrive low-tech version, in which case following the directions should be fine? Excel is a good way to supplement carbon in a low-tech tank. Has this tank always struggled or is it a new issue (I see two different statements regarding this)?
Light is among the most important aspects. Can you tell us what your PAR is at the substrate? You can read about PAR here: https://www.plantedtank.net/forums/1...ml#post1951160
. To find your PAR, you may have to search the forum, for your particular light, to see if others have quantified it. It could be that you have too much light for a low-tech tank.
Your KH, if it is actually zero (test kits can fool you) would probably be better a little higher. I would check your water source to see if you get the same results and let us know what you find. It is not critical to the plants what the KH is (calcium is measured within the GH, not the KH). However, if you actually have no bicarbonates, your BB could be struggling. You don’t need much. Many of us, including me, have KH less than 1 dKH. To raise it, add either sodium (baking soda) bicarbonate or potassium bicarbonate.
Unless you are willing to change the carbon every week (it only last about that long), you may want to try Purigen to help remove the organics that can make algae harder to control. Long term, it is healthy plants that control algae.
You should have enough circulation that the plants can be seen moving from top to bottom, very gently. You should also have the surface of the water rippling (not breaking) well to encourage gas exchange.
Make sure the tank is free of detritus and clean your filter weekly until you get a handle on how often it is needed.
Nitrate is ok. Many of us run in this area, although many of us also run in the 10 ppm area. Given your low bio-load (1 fish), 40 ppm NO3 does indicate that you may be overdosing the Thrive. You may also want to pick-up a PO4 test kit.