Picking up on a couple of issues...
First, nitrates. With soil you will see nitrates, because organics cause nitrates and soil has organics, plus the naturally-accumulating organics from primarily the fish excrement and other minor sources. It is still worth knowing if your source water (tap water) on its own has nitrates (or ammonia or nitrite), but assuming it does not, nitrate between 5 and 10 ppm with soil is not unexpected.
Water changes remove all pollutants, a general term for many different things that can impact fish. With the situation here, I would do at least half the tank once a week, or a tad more, say 60-65%. Test nitrates just prior to the water change so you know where they are at the end of the week, and test the day following the water change to see how much (if at all) they change. Let us know the numbers.
Prime is not a water conditioner I would use except during cycling or if you have nitrite or nitrate in the source water. I am assuming for the moment you do not (your tap water tests will confirm) so there is no benefit in Prime over other conditioners. [Prime binds ammonia (by changing it into harmless ammonium), nitrite and nitrate, but only for 24-36 hours. These will still show in tests even though they are "bound". But not having ammonia, nitrite or nitrate, this is of no benefit.]
Water conditioners. These use chemicals to do this or that. All substances added to the water in an aquarium get inside fish, into the bloodstream and internal organs. It is always best to limit additives to only those absolutely essential. While they may not outright kill fish, they do affect their physiology and/or metabolism, and keeping them minimal is best. Assuming you have chlorine or chloramine or both in your tap water, a dechlorinator that handles these is all you really need. Most will do this, and usually deal with heavy metals too. That is OK, but it is what they may do beyond this that you have to be aware of; some add ingredients (like aloe vera as one example) that do not benefit, and may even harm over time.
That brings me back to Prime. It binds ammonia, nitrite and nitrate, but only for 36 or so hours. This is not necessary in a healthy aquarium with live plants, so it is doing something that will not have any benefit. And how it does it may be detrimental in the long haul, regardless of what Seachem may say. Chemicals do not belong inside an aquarium because they do not belong inside fish. Again, unless specifically necessary for some specific issue.
I use API Tap Water Conditioner. It only does the dechlorinating and heavy metals bit. It is highly concentrated so you use very little, and this is also beneficial because less is getting in the water to get inside fish. If you only have chlorine and not chloramine in your tap water, one drop of this conditioner will dechlorinate a gallon. With chloramine they recommend two drops per gallon. This is even less than Prime recommends. A very small bottle will last you months, even years; a medicinal dropper to measure is a good idea. When I change the water in my 10g tank I change about 60%, which is 5 gallons using the bucket (a 10 gallon tank will not hold 10 actual gallons with displacement by the substrate, decor, etc) and I just add one drop per gallon in the bucket.
