That's an interesting question about dechlorinator. I too only add enough to the water I am adding as what's in the tank is already treated.
As far as I am aware, dechlorinator works straight away and then it's job is done. If the dechlorinator leaves the water I believe the chlorine and heavy metals are taken with it. I may be wrong on that point.
One question is what if you overdose on the dechlorinator?
Over to someone who may have the answer....
Fluttermoth has very nicely answered the question about bucket dosing of the conditioner versus whole tank dosing when using a hose and I completely agree - it's important to dose to the whole tank volume when dosing the tank directly.
The second question above, about overdosing is one I can comment on. In general, other than being a waste of money, there is not much danger in overdosing if one is treating a mature tank. The rub comes for immature tanks (meaning tanks that are less than one year old) in that it is better not to exceed 2x overdosing of conditioner because the N-Bacs don't like it. In particular, if you are in the midst of any type of cycling, you don't want to slow your N-Bac growth from too much conditioner overdose. During cycling, the recommendation would be to dose at 1.5x to 2x the manufacturer recommendation but not more than 2x. Most aquarists with mature tanks past the year mark will just toss in an amount they know is not an underdose and will not worry about it being an overdose.
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One of my tanks is 4m old and one is more than a yr..but new to me (does that make it new?) I have been dosing what i take out of the tank..so if i remove 10L i pour in new 10L then add dechlorinator for the 10L i replaced, im just wondering if this is stalling my cycles?
I know this can sound odd if you are a beginner but if you add the conditioner directly to the tap water in a bucket, then add the bucket of water to the tank then you only need to calculate the 1.5x to 2x dose for the amount of water in the bucket. BUT, if you pour both the tap water and the conditioner directly in to the tank then it is chemically a different situation and you need to calculate the dose of the conditioner to be 1.5x to 2x (but not more than 2x) of the -tank- water volume (not the volume of new water added.)
The reason is complicated to explain: Any fresh declorinated water body with bacteria growing in it will begin to have thousands of types of organic molecules (invisible to you of course) floating around in it and the sodium thiosulfate (or any conditioner chemical) will react with lots of these, leaving less sodium thiosulfate to deal with the chlorine product, which is your main concern (the chlorine being deadly to bacteria and fish of course, in high enough concentrations.)
[Conditioner is cheap insurance against the fairly rare but definately possible event that the water authority shocks your water supply with an overdose of chlorine. Nearly all the experienced aquarists I know use Prime (or perhaps cheap pond dechlor if the tanks are mature) so that even dosing for a large volume is still just a partial capful or a few drops.]
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