Steve,
The degradation follows a pattern that I attribute to life rather than
chemical degradation. Chemical degradation changes in fairly
predictable ways, unless there is a source of catalyst entering the
system, in this case, a biological catalyst is the best explanation.
Here's my rationale:
1. This particular degradation stops when I remove the sample from
the tank, based on repeated measurments over several days. All I do
is transfer water into a vial and cap it.
2. When my tank was CP-naive a year ago, CP settled to a degradation
half-life (t1/2) of about 11 days with my LEDs on reduced power and
about 9 days if my UV sterilizer remained on. The half-life was not
affected by my skimmer. The degradants then were different than they
are now.
3. I treated once within a HT after the 1st failure, but didn't
monitor levels, as I had become overconfident in the T1/2....this
failed.
4. When I began this year, I noticed a new degradate peak, to the
exlusion of all others. The half-life of CP had dropped to 2-3 days.
After repeated dosing over a month, the half life steadily fell until
it was less than 12 hours. I went so far as to dose 20 ppm and
watched it convert completely within 2 days.
5. I took a break from CP for 19 days after my desjardini sailfin
died of what I believe to be toxicity from the high level of the CP
degradation product. Started with a large water change to get the
degradate and salinity down, and tried paraguard (formalin
substitute) along with hypo at 1.009 (calibrated refractometer). This
proved to be inadequate, reducing signs of disease, but never
eliminating it.
6. After this 19 day break, I was planning to switch to quinine and
had ordered "crypto-pro" which turned out to be CP. I decided to dose
the CP, and found that the degradation had slowed down going from a
half-life of ~0.5 days to about 1.3 days. At this point I removed all
remaining sand and base rock (except a small container of new sand for
my wrasse to sleep in). I was thinking the sand might be harboring
the offending catalyst. However, this had absolutely no impact on the
CP degradation rate. Over the next 5 days, the half-life once again
reduced to <12 hours for CP in my tank.
I have been studying the chemical degradation of drugs for 15 years as
part of my career, and this is definitely NOT a chemical behavior.
The only time I ever saw similar behavior was when studying a diacid
salt form about 10 years ago. I scratched my head for a long time on
that one until I realized that the diacid is a food source for life,
and the acceleration occurred faster at the pH and temperature where
microbes thrive.
Regarding where the drug is effective, I believe anywhere other than
the cyst stage from what I've read and observed. If it couldn't work
on an actively feeding parasite on the fish, then medicated foods
would have zero value instead of limited value. When using drugs as a
bath, even if the fish weren't drinking the water, the drug should be
absorbed through gill tissue, which is highly vasularized to speed gas
transfer.....and is known to absorb some drugs from water, as noted in
the Merck vet manual.
I will try ttm as means to QT new fish in the future, but I don't have
large enough tanks to feel comfortable using it for my current live stock...
