Did the LFS give you the numbers? If not it could be that the store just said it was OK and their idea of OK might be 40ppm and you have had a problem all along and not known it.... now it has escalated due to something in the tank.This is the first time nitrate has been a problem. I have been having it tested at the lfs. These last few days I have been testing it myself. Followingr the instructions above, I have been shaking the reagent for 2 min.
I had some sort of dwarf sword and found that it liked more light than I could provide. You thought that your CO2 was high and if it were, then this will not be detrimental to the plants.... fish if it gets really high but you would see signs of them gasping or spending more time at the surface than normal before you would see bodies. If you are not fertilizing, it would help as faster growing plants consume more ammonia. Even just a weekly does of Seachem's Flourish Comprehensive and a few root tabs for the swords would be a big help.The plants are doing OK although not growing very much which I assumed was due to a lack of fert. Nothing has melted. Of course, several are fairly new. The dwarf sword is turning brown which I attribute to the CO2 issues. One of the large valls is being shredded by the fish but the other one looks OK.
I don't know about the filter specifically (personally I would stay away from more moving parts (bio-wheel) than needed) but for the price you might be better going to a canister style filter.I am going to buy new bulbs for the lights so they are brighter. This afternoon I purchased a vacuum for the substrate as well as a new pump/filter, Marineland Penguin 350. There wasn't much to choose from at the lfs.
Unless this not a particularly good piece of equipment, I'll change it out tomorrow.
The while nitrogen cycle starts with ammonia from fish and decaying plants or food in the tank. The more "crap" that you can vacuum off the bottom the less turns into ammonia. The ammonia is either oxidized by micro-organisms (bacteria and/or archaea) into nitrite or consumed by the plants directly leaving no additional nitrites. The nitrites are in turn oxidized by another set of organisms into nitrates, plants have little effect on the nitrites. The less cleaning to catch excess ammonia producing crap and the fewer plants to absorb the ammonia the more gets turned into nitrites which gets turned into nitrates.
Obviously your tank cycle is working well but water changes are needed to eliminate the nitrates on a more regular basis and of higher volume. I'd say 50% per day until this gets settled.
Your fish load is not that huge that they should be that much of a concern, although that depends on the amount of feeding as the more they eat the more crap is produced.... etc. You may consider putting more plants in to help catch the ammonia up front but if you get on top of the nitrates and don't want any more plants, it will be fine.