With the exception of very uncommon forms of algage which occur in the
ocean (Red tide, caused by Diatoms, not algae). There is nothing to
worry about with green algae occuring in a bird bath. The enzyme that
you purchased is very likely to be barley extract, or some type of
algaecide (algae poison).
Most toxic critters are formed from anaerobic (oxygen less)
environments, such as stagnant water with lots of debris in it (you
basically get an outbreak of funii and rotting, fungii can be toxic).
The presence of green algae is usually a good indicator that there is
oxygen in this environment, and rotting is not the case (However, once
the water temperature exceeds 100F, there is no oxygen in it).
Algae will be caused by an excess of nutrients, and an excess of
lighting. Usually the limiting factor here is phosphates in the water.
Most algae problems can be cured by removing phosphates from the
water. This probably isn't very practical in your case.
I wouldn't be suprised if birds showed a preference for clear water,
but there is nothing to worry about. If you're truely worried, add a
very small amount of bleach to the water. A good, small mouthed,
bullet proof snail is Melanoides Tuberculata, commonly referred to as
the 'Malaysian Trumpet Snail'. These have a very hard shell, which
would provide protection against the birds, also, when the conditions
get rough, they close up into their shell, and can withstand quite
harsh conditions. Also, Petsmart sells a large 'Apple Snail', which is
a voracious eater, and cannot reproduce on it's own.
Lastly, perhaps you should consider an epoxy like painting (or just
epoxy) on the bottom of your bird feeders to give it a smooth surface?
Every epoxy that I'm aware of is non toxic when fully cured, and
provides a water proof, extremely durable, smooth surface, which would
be easier to 'wipe off' than a concrete surface.