My favorite read on Excel/Glutereldehyde, with the majority of the information (except for comments) from 2 folks who I absolutely respect in the hobby.
https://www.sunkengardens.net/blog/...-liquid-co2-and-the-dangers-of-glutaraldehyde.
Normally, the 3 ways to remove Nitrates from teh wqater are.
Chemically, through adsorbing through a filter pad or whatever, which is seldom something I would consider.
Using live plants, and them growing successfully will use nitrates/nitrogen compounds as nutrition
or Water Changes which will put the nitrate level somewhere between where it is in the tank, and where it is from the tap.
Nitrates will come from
Being added chemically, like Thrive, or Seachem Nitrogen, or whatever aquarium fertiliser you use.
Being added through water changes if your tap water contains them.
Fish/Animals living in it.. it's a biproduct of using food, you said you were overstocked, and this can be related to the bioload of your tank.
It was a used tank, how well was it cleaned? any organic matter will add to nitrates also. I'd suggest a good cleaning of filter media, and gravel vacuuming of the substrate, check around any hardscape/decor to make sure there isn't something that really should be removed.
I'm gonna say that you should
- Test your aquarium water, and your tap water.
- list the contents of your aquarium livestock, plants, fish, size, etc
- Share a picture of your aquarium so we can try to determine the actual plant mass that's in there
- Describe your water change schedule, how often, and how big.
- Maybe describe the equipment you are using, light especially
I would not worry about the diatoms, they should go away by themselves and are likelihood due to moving substrate around or changing it, which is normal even when moving an established tank. For the green algae, I would likely decrease the amount of light, put it on a timer and have a schedule where the light is on for maybe 8 hours a day, or 2 sets of 4 hours. make sure that there is no natural light hitting the aquarium, this will help plants or algae grow too, just the same as if you had a bigger light on top of the aquarium.
Hope that Helps.