How do you remove nitrates normally? A PWC
will mearly lower the level by 10% if that is you WC
schedule. Do you have a skimmer or macro algae. Those are the only real methods to lower nitrates significantly (other than massive radical water changes) other than using resins or the such.
If you rely solely on water changes to lower nitrates, your nitrate level will rise until the amount you remove weekly is the same as how much protien (food) you add per week.
1 tsp (5ml) pure protien (food) added per week into a 400l system (75+30 * 3.8) is
.005/400 = 0.0000125 L/L or 12.5 ppm
Removing 10% per week, or more correctly, leaving 90% behind per week, your level will increase until your converted protiens (nitrate) are at a whopping 125ppm.
Yeah water changes.
So you need to remove 5mL of food per week or 12.5ppm nitrate. Now denitrifying bacteria will remove a small portion of that 12.5ppm, but they still need significant help.