Little hard to find, but this is the best information you will probably get,
"Fluoroquinolones (FQs) are fully synthetic antibiotics that are widely used in humans, animals, and fish (Grave et al., 1999; Le and Munekage, 2004; Samanidou et al., 2005; Kemper, 2008). The antibacterial mechanism of FQs is based on inhibition of bacterial DNA gyrase or topoisomerase IV, which are enzymes essential for DNA replication. First-generation FQs are piromidic acid, oxolinic acid, and nalidixic acid, which were widely used in aquaculture in the 1970s in Japan. In tropical Asian countries, oxolinic acid still appears to be one of the major drugs used (Grรยคslund et al., 2003; Le and Munekage, 2004). Second-generation (ciprofloxacin, CIP and norfloxacin, NOR) and third-generation (levofloxacin, LEV and its enantiomer ofloxacin, OFL) compounds are used in hospitals and animal husbandry in Indochina (Takasu et al., 2011).
The FQs are photo-degradable, with a half-life in pure water of 105 and 90min for NOR and CIP, respectively (Burhenne et al., 1997). However, FQs in the environment are relatively stable in water and sediment (Kรยผmmerer, 2004; Le and Munekage, 2004), which might be due to sorption onto particulates (Nowara et al., 1997). Lai and Lin (2009) reported that oxolinic acid and flumequine could be retained in sediment for 9.5"โ15 and 3.6"โ6.4days, respectively. Such long half-lives in the environment pose a selective pressure for environmental bacteria. Although the bioavailability of antibiotics is suspected to decrease upon adsorption on clay and humic substances, no supporting evidence has been reported."
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3283837/
In general what this is going to mean is that you will need to keep doses up pretty high in a BB tank to make sure that the photo-degradation doesn't rid the tank of anti-biotics. This does not however account for absorbtion by the animal tissue or retained water in the animal not exposed to light.
To make anti-biotics effective the level needs to be maintained high enough for the bacteria not to build up a resistance to the anti-biotic. If the bacteria is not killed off by the original few doses another source will need to be used in order to remove the bacteria as they will evolve with resistances to the previous anti-biotic treatment.
Using Minh's method I would perform a daily water change adding as advised 25mg/3.79L (lets stick to metric measurement methods)
Just keep in mind that if you are going to skip a day of a water change, I would add additional anti-biotic daily to ensure the level stays somewhat consistant in the hospital tank.
Hope this helps.