I have a heavy planted tanks (well several of them) with 1 or 2 pairs of fish (for sure not enough waste for fertilizer) .........I have heard good things about seachem products .....need something that won't kill the fishes
However, if you've only got a pair or so of fish in each tank, it makes me wonder if they are big fish? And therefore a big tank? Obviously, it would get mighty expensive to use Excel in a large tank; Co2 injection would be the more feasible option.