> Noticed the presence of the distinguishedly-smelling cyanoalgae/bacteria in
> my 100g tank. Threw in 2 tablets of 250mg erythromycin which solubilized
> within minutes. Water remained very clear.

> Inference:-
> Erythromycin, being an antibiotic, must have affected the biological
> filter. The effect peaked roughly from the 14th day. It took about 6 days
> for the biological population to recover enough to carry on its job of
> water polishing and filtration.
> Recommendation:-
> Since only 500mg of erythromycin were used (unlike earlier suggested dosage
> in the mails)in the 100g tank, it can be seen that its effect on the
> cyanoalgae was astonishing. Hence to avoid further detrimental effect on
> the b/filter, use only the minimal. I will try out only 250mg should there
> be a future outbreak of the cyano stuff again.
> APD members are welcome to comment.

I hate to pee on the parade, but IMO, the use of antibiotics in this 
fashion is not the way to go.  You are getting more cyanobacteria than 
you want because there is a problem with either the chemistry or the 
biology of the system.  

If I had to venture a guess, I'd say that your tank is probably short on 
some microbes or very small arthropods that eat cyanobacteria.  Your tank 
may have more phosphate than it needs.  Adding erythromycin doesn't take 
care of either problem.  It will however eventually breed resistant 
cyanobacteria and bacteria.

Your analysis of the aftermath is essentially correct.  The cyanobacteria 
died, their nutrients went into solution, where that bacame bacteria 
food.  There was a bloom of bacteria, prehaps accompanied by a spike of 
ammonia, which your plants probably helped with immensely.  Eventually 
the system comes back under control.

If anyone else tries this, you should watch the dissolved oxygen 
concentration, especially at night.  Based on my experience with 
antibiotic use in reef aquaria (where there is also a bacterial bloom) if 
the bacterial bloom is sufficiently intense, you may lose some fish to 
anoxia at night.  Theoretically, you could crash a tank this way, 
although I doubt that will happen very often in practice.

The cyanobacteria always comes back in reef systems if something is not 
done to address the underlying problem.