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When I searching about a drop checker .I can't find much info .Can some one explain how this devise works .I understand there is a agent poured in a tube ,and its put in the tank .Does the agent change colors ? How long will the agent last ?Can someone explain.
 

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The way it works is by CO2's effect on the pH of a standard solution, which is viewed using an indicator. When CO2 dissolves in water, it forms carbonic acid, lowering the pH. If you start with a particular solution (4 dKH solution) that is buffered to a specific pH, the small changes in pH that dissolved CO2 creates cause fairly large changes in the color of an indicator (bromthymol blue, the blue/yellow indicator commonly sold in pH test kits.) The drop checker is set up so that CO2 in the solution and the CO2 in the aquarium are related, even though the water doesn't mix between them.

Heres a crappy drawing.

drop checker.jpg

As the amount of CO2 in the tank increases or decreases, the amount going from the tank to the checker increases or decreases, which causes the color of the checker to change. You're generally shooting for a lime green checker, if everything is set up right and you're using the standard solutions. These work well, with the caution that they are fairly slow to respond to change. A shift in CO2 concentration in the tank takes time to translate into a shift in CO2 concentration in the checker because of the roundabout path it must take to get there.

Hope this helps.
 

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Also, don't bother with getting a drop checker with the indicator solution. Just get the "container" if you can. You can use the API pH test kit solution too. Regardless, you need to order a 4dkh solution to make the testing accurate. Unless it is stated, most drop checkers do not include the 4dkh solution. I got mine from Ebay though after I bought a drop checker with its own indicator solution. For some reasons, it turns yellow first with the included indicator solution. I use the solution from my Tetra pH test kit instead.
 

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You can get the checker and solution fairly cheaply on the swap and shop here. Can't remember who it is, but there's always at least one person selling them. The price is very reasonable.
 
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