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Steve Parry  wrote: <<<< (...) I would like
to know what "happens" to anti-biotics in a closed system like an
aquarium. If, for instance no carbon is in the filter (does carbon
remove it anyway?) and only small water changes are done, does the
anti-biotic remain in the system, and does it remain "active"
What ARE anti-biotics anyway? Are they "active"? Alive? How do they
work? Do they all work in the same way?>>>>>

Thanks for the tips about the "sunset" hygro.
The questions you asked are quite extensive. Antibiotics are substances
produced by certain microorganisms, that may inhibit  or even kill other
microorganisms. The term is however applied also to similar compounds of
man-made origin. Each compound has its own characteristics in term of
stability, mechanism and spectrum of action: to generalize here is quite
difficult. Similarly, the metabolism and elimination of each compound
varies by animal species and by other individual characteristics. The
bottom line is that there are situations when their use is likely to
produce a benefit that outweighs the risks of toxicity, and other
situations when their use is worthless or even likely to make the
problem worse.
Antibiotics may be introduced in the aquarium dissolved in the water,
mixed with food or, more rarely, given parenterally or by forced-feeding
to the fish. After its introduction, several events may take place.
Within the organism of the targeted fish, the compound is in most cases
metabolized and eliminated in a different form (which may or may not be
inactive), but it may also be eliminated without any metabolic
processing, or it may accumulate. Same type of possible scenarios for
the multitude of other living organisms in the aquarium. Antibiotics may
also precipitate, bind to organic and inorganic molecules in the water
or in the substrate (activated carbon is one of those substances),
degrade spontaneously or under the effect of light and temperature. The
treatment recommendations available in the literature attempt with
various degrees of success to account for all these factors. Each
transformation has the potential to produce by-products with undesirable
properties, but occasionally metabolic processes are used to enhance or
activate a given compound (which is called in this case pro-drug).
Plants, as far as I know, have not been studied in terms of the role
they may play in degrading antibiotics. I read of anecdotal reports of
major plant crashes in association with antibiotic use, as well as of
reports of medication of planted tanks without any detectable negative
effect. I am not aware of specific contraindications based on reliable
studies (excluding copper and algicides). Maybe other people on the list
can share some useful information. 
The overall practical conclusion for planted tank owners is, in my
opinion: 1) quarantine all fish and, in some cases, prophylactically
disinfest them against specific common infectious diseases. 2) get a
good book on fish diseases, and become familiar with it before trouble
occurs. 3) use medications as directed by the book (the manufacturer's
instructions are often very poor) checking carefully first for any
contraindication, including presence of plants. Every time the treatment
can be administered in a separate bare hospital tank, do it if you do
not have reliable experience on the compatibility between treatment and
plants. If a planted tank must be treated, do it when you are around to
detect any early sign of trouble.