Hello I am Szabadasz
I recently have had trouble with my nitrate
I checked it today along with my ph and iron
It was high so I took out 25% of my water replaced it
Add water conditioner then checked my nitrate fixed my iron and re positioned my plants
After I checked my iron again and noticed it was a litle too high so I took out 10% of my water in my 20 gal tank replaced it checked my ph and iron they were good then I checked my nitrate it was to high. Other info on my tank is I have 2 neon tetras 3 plants 12 longfined leopered danos 4londfined zebra danos 1 leopered dano 1 zebra dano 8 gupies 4guppie fry and 1 dead adult gupie removed shortly after death before tests done. I aso have many small natural snails that were attached to my plants wene bought. Ihave not yet ever done cycle or waste control or have a under gravel filter.mY TANK IS ABOUT 2 AND ONE HALF MONTHES OLD WITH PREVIOUSE LY USED GRAVEL FROM A YEAR OLD TANK I have a 20 gal power head filter with a new cartredge in it. If I get an under gravel filter to help remove exes waste and plant debrese and start to use cycle and waste control will that help with my nitrate problem?

Undergravel filters are really no good.
They suck all the stuff that would be removed by filter cartridges under the gravel for it to stay.
Are you using an iron supplement?
Nitrates build up in a cycled aquarium when not enough water is being changed out.
Try doing a 25% water change daily until the nitrates are in the safe range, then one water change every 2 to 4 weeks to maintain safe nitrate levels.
There is no need to use cycle or waste control if you do the water changes.
Good luck and happy fish keeping.