the fact that you have ammonia/nitrite readings is probably due to two things:
1) Low levels of beneficial bacteria. This is probably due to washing the filter with tapwater, in addition to the filter possibly being inadequate. Also, as I recall, higher temperatures cause the metabolic processes of bacteria to slow down, slowing their uptake of ammonia/nitrites. Again, leave whatever filter pads you have in the tank, I wouldn't even rinse at all! Might help to add some aged, colonized gravel in the filter too, speed up the process.
2) Poor plant growth. Over the past months, have the plants GROWN much? Or are they merely "sustaining"? Seems to me that they need more light, then think about ferts/CO2. When you provide these things they will eat up that ammonia/nitrites first thing! I don't have experience with AH supply, but another strip light would probably help (you didn't say what light you DO have?) For a 10 gallon, would consider adding Excel for carbon source (but maybe not if doing 50% PWCs)
Canister: seems overkill for a 10-gallon, but maybe they make some small enough. . . check out ebay for sure.
Also, the hot temps would contribute to fish deaths.
HTH, and good job on the water changes! That will probably help you the most at this point.