A couple things stand out from the answers you provided in post #10. First, what is the additive referred to as "slim coat"? It might also help to know what the "dechlorinating filter" is/does.
The other issue is the water changes and nitrates level. Nitrates at 10 ppm is not dangerous, but the fact that they fluctuate does suggest that water changes should be more substantial and regular (weekly should be fine) to avoid any fluctuation from day to day or week to week. Water changes are intended to keep nitrate and pH absolutely stable from one W/C to the next. If nitrate is increasing, then changing more volume once a week should solve this. Of course, we don't have the lower nitrate number, but if for example it is say 0 to 5 ppm the day following the W/C, then it should not increase to say 10 ppm before the next W/C.
The other thing is that nitrate is only one test we can use, and not the only reason for a W/C. Change the same volume regularly, even if nitrate test results show no increase. There is a lot of "crud" in the water that is best removed, and for this we cannot measure, we just know it is there because it is part of the biological processes occurring in any fish aquarium. And you do have some larger fish that obviously create more of a biological impact than much smaller fish do.
The other issue is the water changes and nitrates level. Nitrates at 10 ppm is not dangerous, but the fact that they fluctuate does suggest that water changes should be more substantial and regular (weekly should be fine) to avoid any fluctuation from day to day or week to week. Water changes are intended to keep nitrate and pH absolutely stable from one W/C to the next. If nitrate is increasing, then changing more volume once a week should solve this. Of course, we don't have the lower nitrate number, but if for example it is say 0 to 5 ppm the day following the W/C, then it should not increase to say 10 ppm before the next W/C.
The other thing is that nitrate is only one test we can use, and not the only reason for a W/C. Change the same volume regularly, even if nitrate test results show no increase. There is a lot of "crud" in the water that is best removed, and for this we cannot measure, we just know it is there because it is part of the biological processes occurring in any fish aquarium. And you do have some larger fish that obviously create more of a biological impact than much smaller fish do.