Here is a pic of a drop checker, basically what it does is help you measure how much Co2 is in the water of your tank by adding a solution of 4kH water with some of their indicator liquid (amount is stated in the instructions) into the little vile so it ends up in ball area if that makes sense...
Then once setup in your tank with Co2 running, it will slowly change the color of the indicator liquid inside to either a blue / Green / Yellow depending on the level of Co2 inside the tank...
Blue being too little Co2, green being around 30ppm and yellow being too much, although these kits do come with a color chart as well for you to check levels and let you know exactly what the colors mean...
Also, its a good idea to place the drop checker on the opposite end of where you place your Co2 diffuser to get the most accurate results, i.e. if the diffuser is on the right of your tank, you place the drop checker on the left...
As for brands, i guess any from a reputable brand are okay, i personally used an ADA one just because that was all they had at the store when i bought mine, but there are cheaper alternatives that work just as well...
I should also note, since brands like the ADA one didnt mention that you need to pre mix 4kH water to mix the indicator liquid with in the english instructions, so i would search online on how to mix the water yourself, i think it was using something like 6grams of baking soda to 1 liter of DISTILLED water to make the water 4kH, but again double check as i just pre-mixed a bottle and stored it for when i need to change out the solution in the vile...
Hope that helps a bit, and if you have any more questions, please ask...
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