You actually need to re evaluate the set up of the tank as well. Given the tank height, hex tanks are notorious for low pH problems due to the lack of surface area. It becomes even more important to have brisk surface aggitation and as much uncovered top space as possible to prevent the buildup of CO2
. The fact the airstone improved your pH would support that. I can see by the picture you posted there does seem to be a lack of water movement apparatus.
As for the filter being added, it is by no means a contributor to the problem. If anything, it will help improve conditions some. Just be sure you clean/replace the media frequently. If simpley cleaning, rinse everything in SW
not fresh. This will ensure a good majority of nitrifying bacteria survive. You can also use the return from the canister to hook up a spray bar. I find especially in a smaller tank, spray bars are very efficient and improve overall water flow.
Another issue to consider is lack of de
nitrifying areas. If you like the look of the tank without much LR
, you need to improve nutrient export a great deal or high nitrates will always be a problem. A good skimmer, carbon use in the canister and/or a refugium for macro algae harvesting. If you don't mind the look of the LR
or even base rock as a cheaper alternative, it will greatly improve the living environment/water quality.