First, remember turning any flourescent bulb on and off rapidly shortens it's life. So avoiding this is important. By reducing the 'duty cycle' (percentage of time the bulb is on) will lengthen bulb life, as long as you avoid short cycling, as above.
I would try 12 on/12 off, and see how that works. Alternatively, running it for 1 or more days each week might work, as well.
Some considerations are-
"The time it takes for a bacterial cell to grow and divide in two is called the doubling time. When nutrients are plentiful, the doubling time of some bacterial species can be as short as twenty minutes. However, most bacterial species show a doubling time between one and four hours."
Read more: http://www.biologyreference.com/Ar-Bi/Bacterial-Cell.html#ixzz16bXCG7Pu">Bacterial Cell - Biology Encyclopedia - cells, body, human, process, animal, system, different, organisms, chromosomes</a> [IMG]http://www.biologyreference.com/Ar-Bi/Bacterial-Cell.html#ixzz16bXCG7Pu">http://www.biologyreference.com/Ar-Bi/Bacterial-Cell.html#ixzz16bXCG7Pu</a>
For cryptocaryon (major marine parasite encountered-IME)-
<span style="color: black">"The free swimming tomite has less than 12 hours to find and invade a host fish, otherwise it will exhaust its energy reserves and die."</span>
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<span style="color: black">[IMG]http://www.fishvet.com/Cryptocaryon.htm">http://www.fishvet.com/Cryptocaryon.htm</a></span>
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<span style="color: black">This why I would choose 12/12 cycle.</span>