I figured I would let you know of my recent experience with a gluteraldehyde (Excel substitute) overdose. I have been battling BBA in my tank due to the absence of CO2 for a 2-3 week period. I decided to make up a 4.5% gluteraldehyde solution from 25% stock. I started dosing 2mL a day, which is roughly equal to a double dose of Excel (assuming Excel has a 1.5% gluteraldehyde concentration). When I didn't see any immediate results, I slowly upped the dosage to 3mL (3X Excel dose).
Still nothing, so I got frustrated a week later and dosed 5mL one day, equal to a 5X Excel dose. I left for work and come home to a nice, sticky kind of froth on the top of the tank. My fish were acting erratic, almost like a CO2 overdose, gasping at the surface. I immediately did a 75% WC to reduce the gluteraldehyde concentration in the tank.
That was a week ago, and my tank is still looking horrible, with melted stems and such. Overall, I lost 1 tiger barb, and probably permanently injured the other fish in the tank
(peppered cories, otos, and BN pleco). My Hygrophila corymbosa took the biggest hit, with almost every single leaf melting off (probably 75% loss). The wisteria was also hit hard, I estimate 60% loss. Brazilian pennywort is just now showing the effects, where 25% of leaves have melted. Rotala was a 100% loss, but wasn't that healthy to begin with, so I cant blame it solely on the overdose. Bacopa looks to be a roughly 40% loss. My Amazon swords have shown no ill effects thus far. Almost all of my floaters (giant duckweed, red root floater) died or started to brown. BBA has not reduced significantly, so the only damage I did was to my plants and livestock.
I guess this is just a warning to others out there to not be impatient and do something stupid like I did. I really should have known better, as I have worked with gluteraldehyde extensively in my research. Once I get the tank healthy again I will dose 2X max to beat back the residual BBA.