What I would say is killing your plecos is a combination of things, although some factors have been left out that could also affect this. The things killing them, is a high stress situation (water changes, low O2, high temp), O2 levels possibly being too low, and temperature being too high.
x 2 with a few additional caveats ............
Sterling ............... for a starter we need to know the pH value of your tap water, as well as if it is treated with chlorine, or chloramine, and at what level. (ppm or mg/l) 1 capful of Prime treats 50 gallons of tap water, but that doseage rate is based on how much chlorine or chloramine is used by your local water supplier.
The only way that one can truly know how much Prime (or whatever) to add to their tank is by knowing what quantity of chlorine or chloramine is being added by their local water treatment facility, or more importantly, what the rate is as it leaves the facility for end use. Without that data, it becomes purely hit or miss.
Seachem Prime states 1 capful per 50 gallons, but that's for 4 mg/l chloramine, and/or 5 mg/l chlorine.
In my case I can safely use half that amount, as I'm treating for 2 mg/l chloramine.
As far as tempertaure ......
http://www.seachem.com/Products/product_pages/Prime.html
If temperature is > 30 °C (86 °F) and chlorine or ammonia levels are low, use a half dose.
BTW - water conditioners in their basic form are reducing agents, and all of them react fairly instantaneously, although they can continue to react on contact depending on various factors, such as organic levels, pH values, etc.
With regards to treating for the entire tank volume, when filling directly from the tap. (such as Seachem suggests)
That also depends on a number of factors. When chloramine tap water is treated with products such as Prime, Safe, or ClorAm-X, the chlorine/ammonia bond is broken, resulting in a certain amount of free ammonia (NH3) that needs to be bound or reduced into a safe non toxic form. The toxicity of free ammonia is highly dependent on both temperature, and pH, so this can vary GREATLY from one hobbyists tank to another.
A good read on the toxicity of ammonia can be found in the following article posted on the krib.
http://www.thekrib.com/Chemistry/ammonia-toxicity.html
Those with only chlorine (as well as many that have to deal with chloramine) treating for the entire tank volume when filling directly from the tap may in fact not be neccessary.
For chlorine only, it's a non issue.
For those with chloramine it may or may not be required. In my case pH of 7.8-8.2, water temp of 80-82F, chloramine value of 2 mg/l, I can safely treat for only the amount of new water added directly from the tap. I have tested for both chlorine as well as free ammonia, and the values are 0.0 throughout the fill, and after. (water conditioner added before & during the filling of the tank)
I recently contacted Dr. Greg Morin, the Chairman/CEO of Seachem labs, and here is a portion of our conversation on this very subject.
I was wondering if you could explain the reasoning behind the instructions by Seachem for treating the entire tank volume (when using Prime/Safe), if one is treating the new water while refilling the tank. "May be added to aquarium directly, but better if added to new water first. If adding directly to aquarium, base dose on aquarium volume. "
Personally I typically add the conditioner (Prime/Safe) based only on the water volume being replaced, to a 1 gallon jug, and then slowly add that mix in stages as my tank refills. Is this not adequate?
Greg's response:
"some people prefer to just add the new untreated water directly to the tank... if they do that then we recommend the amount of Prime they add be based on the total gallonage of the aquarium rather than just what they added. The "extra" amount speeds up the rate of removal."
I then asked: Is the reaction time based on pH, or any other factors?
I was curious about pH being a factor as the makers of ClorAm-X state:
"At low pH's this reaction proceeds slower than at pH's above 7, but in practical terms the reaction proceeds quickly enough to provide complete ammonia removal in an hour or less."
Greg's response:
It would be influced by pH although I'm not sure if the differences we see in an aquarium would contribute significantly to the time scale at a level where it would be noticed. But the reaction is one that produces H+ so higher pH would tend to favor the reaction although I'm not sure if kinetically it would have a noticeable effect.
But, at the end of the day, if what you are doing works and does not cause any problems then it is ok. Our recommendations are meant to cover a broad range of users and we tend to prefer to err on the side of being overly cautious.
Greg Morin
--
Gregory Morin, Ph.D. ~~~~~~~Chairman/CEO~~~~~~~~~~~~~~
Seachem Laboratories, Inc.
www.seachem.com 888-SEACHEM
With chloramine treated tap water, here's my personal take on the subject ..........
When adding new water directly from the tap, and dealing with certain species of fish that are known to be overly sensitive to ammonia, or young fry/juvies of some species, it might be prudent to treat for the entire tank volume, and not just for the amount of new water being added to the tank.
Depending on the species (and perhaps size/age of the fish), the volume of water being added, the overall water circulation in the tank, the temp of the tank water, and even the pH, may determine what is safe for the fish, and/or what might be causing the tank inhabitants long term stress.
What might be considered safe for an adult Oscar, might not be overly safe for a 2 day old Discus, or a marine invertebrate.
When chloramine tap water is treated with products such as Prime, Safe, or ClorAm-X, the chlorine/ammonia bond is broken, resulting in a certain amount of free ammonia (NH3) that needs to be bound or reduced into a safe non toxic form. The toxicity of free ammonia is highly dependent on both temperature, and pH, so this can vary GREATLY from one hobbyists tank to another.
HTH
