UPDATE:
Retested the water. Nitrates are still at somewhere between 40-80ppm (maybe even higher)! After 150% worth of water changes and having both filters cleaned in the last two weeks, as well as having the gravel vacuumed it is still up that high! What am I supposed to do now? Maybe retest tomorrow after it has settled for longer? Let me know what you think.
Do the separate series of tests Steven suggests in post #13. I expect his thinking is that there is likely some interaction causing the nitrates and you need to find out what it is. Narrowing down the options like this often does this.
I also agree on the vitamin additives being purposeless. Missed that previously, sorry. When some additive is not needed, there is no point in adding it, as the fish won't appreciate it and there can be unsuspected interactions within the complex chemistry and biology of an aquarium.
Byron.