nothing wrong with doing fish in. fact is, anybody could have lost those 2 fish no matter their approach to getting a tank going. sometimes fish you buy just dont make it.
i think you have multiple issues. i would start with eliminating the moonlight , and maybe reduce the regular lighting a tad, but i would also say you need to really up the water changes until you get it under control. manual removal of what you can would be my very first thing though.
im no engineer, but if it were me, and i wanted a stand more than a few inches wider than the tank, i would double up the front and back top 2x4's, just to be sure they didnt sag over time.
you arent wasting money, as the fertilizer in the root tab is still going into the water column. i would say dosing remains the same, as it all still goes into the water.