Killed my fish trying to kill algae


Prolific Poster
Jun 23, 2005
I had a major catastrophe in my tank today - 12 dead discus and assorted tankmates for no apparent reason. I would appreciate some help in figuring out why.

I have a 180 gallon tank, moderately to heavily planted, with all appropriate fertilizer levels and CO2. I added a new piece of driftwood to the tank 3 weeks ago, and it started to grow hair algae on the top branches that were close to the surface. This morning I used a turkey baster to inject some hydrogen peroxide directly onto the algae. Total peroxide used was 3-4 ounces of standard 3% solution. About 2 hours later, my fish started dying off one by one. An hour later I had lost 12 discus, around 10 cardinals and one blue ram. I am left with 10 more cardinals, 3 cories, 10 or so ottos and 2 gouramis all of which seem unaffected.

I don't understand what happened.

Two possibilities:
1. The peroxide was actually much stronger than 3% even though the bottle said 3 percent. It was a no-name brand that I picked up in my local supermarket.
2. I later read that peroxide interacts with ferrous iron to create some really nasty compunds that destroy all organic tissue. My iron level was around .3ppm and I had added more 2 hours before I put in the peroxide.

Either way, if the water became toxic enough to kill all the discus, why didn't it kill everything? Any thoughts?

Tom Barr

Staff member
Jan 23, 2005
Re: Killed my fish trying to kill algae

They can be more sensitive than the other fish.
Some fish at ther borderline might die, whereas the stronger fish, or the ones that got lucky enough to hang out elsewhere were spared.

Realize that H2O2 disspates and breaks down in about 1 hour.
A largen shock load moving through a large tank is not the same as a well mixed batch going through 10 gal tank also, the same response time issues prevail with CO2 dosing etc.

I'm never keen on using things like H2O2 nor other algicides.
This is rare, but no less meaningful to you at this point.
I'm sorry for your loss.
I never use it except see if it's useful for other folks and see how it works etc. I have less need for such things, frustrated folks often do ansd rationalize it. I'd rather spend a bit more time pruning and cleaning. The tank looks better rather than the quick fix.

Seems 3-4 oz should not have done much, not sure about the Fe though, recall it's chelated also.

Tom Barr