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Can someone clarify how to use Prime?

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I was reading a few things based on some advice I received here over the last week or so after issues in my tank. I was under the impression you add Prime based on how much new water you're adding to the tank after your water change. Example, I take 20g out, so I dose 2mL for that 20g, even though my tank is 55g. That is what I have been doing all along. Two articles I read said you dose for how many gallons are in your entire tank, not just what you're replacing. 

So off I go to look at the bottle of Prime. Directions are as follow:


Use 1 capful (5 mL) for each 200 L (50 US gallons) of new water. For smaller volumes, please note each cap thread is approximately 1 mL. May be added to aquarium directly, but better if added to new water first. If adding directly to aquarium, base dose on aquarium volume. Sulfur odor is normal. For exceptionally high chloramine concentrations, a double dose may be used safely. To detoxify nitrite in an emergency, up to 5 times normal dose may be used. If temperature is > 30 °C (86 °F) and chlorine or ammonia levels are low, use a half dose.

When I do my water changes I use a Python, the water goes right into the tank from the sink, so I add Prime right to my tank. Should I therefore be dosing for 55 gallons (my tank size) regardless of how much water I am changing/adding back in?

I feel incredibly stupid for even asking this, but I really want to make sure I am doing this right.


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On 5/26/2022 at 8:14 PM, Goldie Blue said:

@nabokovfan87 Sooooo... since you have been following both of my recent posts, could any of the issues with my tanks be because I have been under dosing the tank with Prime for over a year? lol. The fin rot in my bettas, even though the tank parameters were fine etc? Or is it not that detrimental? 

I honestly can't say. It really didn't help, but I can't imagine that it's a simple "this caused this" with the issue you have. 

I think fixing the filter, fixing temps, fixing prime, fixing all of those things together, will help!

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It all comes down to how much water you're treating.  If you change 20 gallons but store that 20 gallons in a barrel or something, then you only need to treat that 20 gallons while it's in the barrel. If you're treating 55 gallons after a 20 gallon water change, you're still treating 55 gallons even though just 20 of it is new. And other than cost, you don't really lose anything buy adding more than necessary and it's better to be safe than sorry.

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