Excel used properly is not toxic to fish or plants. I have been using it in multiple tank for 20 years. I have severl large clown loaches I got at 4 inches in 2002 which have always been in a planted tank thathad excel added.
I have bred DD black angels. assorted corys, BN plecos, montezuma swords, betta imbellis, rosy barbs, P. nicholsi, Farlowella vitatta, Hypancistrus L450 and contradens, cherry shrimp, danios (zebra, choprae and roseus), Pseudomugil furcatus and gertrudae all in planted tanks dosed regularly with Excel. The fish all threw healthy offspring.
I am not suggeesting you get SAE. But to get them to eat BBA you gave to stop feeding them anything else at all. I had am outbreak shortly after setting up a hight light pressurized CO2 added tank because I failed to use teflon tape on all the connection in the CO2 system and it all leaked out feast and it took me a few days to realizre this. The result was nasty BBA in the dwarf hairgrass. I gathered some SAE from other the tanks where I had them and put them into the one tank. I stopped feeding that tank and they ultimately cleared the BBA. I did not add Excel to the tank that got the added CO2
(The dark dots in the hairgrass are BBA.)
Re Excel. If it were harmful to fish and most plants, it would have been taken off the market long ago. SeaChem would have been at risk and would have had their reputation go down the drain.
I do not overdose excel- in fact I probably underdose it. I add 3 ml/10gal. of water after the weekly water change in all 7 currently planted tanks. (It used to be 12.)