Depends on your
tank...... for a 20g, I'm guessing you don't have a sump w/refugium, so pwc
's are going to be your only method of excess nutrient export. Test nitrates before the change & see how much they need to come down..... For example, if you test at 10ppm and want to cut them in half, then a 50% water change is in order. Test them again the following week and see what they have risen back up to. If they have risen back above
10ppm, then you'll either need to up your weekly percentage or do twice a week.... if they haven't come back above 10ppm, then you know you're keeping up with the waste being produced and may be able to cut back the amount.