A lot of people have been asking various questions on Drop checkers, and it is quite hard to write up about them in a short answer, so here is a complete guide to the Drop checker.
A drop checker (or a permanent CO2 test) gives us a constant reading of the CO2 levels in the tank.
We fill the drop checker with either 4dkh, or 5dkh solution (4dkh being the more popular), and add 3 drops of bromothymol blue (Blue liquid which can be found in low range ph test kits). It is important that you use 4dkh or 5dkh solution, DO NOT use aquarium water, as this contains other acidic substances, which can affect the reading (hence why the ph/ kh CO2 tables are inaccurate!)
As the CO2 gasses off from the aquarium into the drop checker, the ph lowers due to the carbonic acids formed by CO2 when it dissolves in water. When the drop checker turns from blue, to green, this indicates a pH of 6.6, and because we know the solution is 4dkh, this tells us we have approximately 30ppm of CO2 in our tank water.
If you are using a 5dkh reference solution, then a pH 6.6 (green drop checker) will indicate 38ppm.
4dkh reference solution is available from here
Bromothymol blue can be found here
A cheap Drop Checker is available from here
Size, shape, price whatever does not affect the performance of a drop checker. And as you can see with the one above it comes supplied with bromothymol blue.
The 4dkh solution may look as though it is expensive, but with the pictures below, I used 1ml to get half the ball full! So changing that weekly will last me 500 weeks/ 9 years (£1.11 per year!) Obviously some drop checkers are larger than others. You can change the solution every few weeks if you like, but I prefer to do it weekly.
If you want, you can always make your own reference, but make sure you have accurate scales (0.01g) and use very accurate measurments:
Add 6g of pure Sodium Bicarbonate to 5l of DeIonised water (DI) to give you a solution at 40dKH.
Mix 100ml of this solution with 900ml of 'fresh' DI to give you 1l of 4dKH reference solution.
This obviously makes a lot, so you can sell some on (providing it is accurate).
You can scale this down; add 1.2g for every 1L of DI water, as this will always give you 40dkh
You always mix 100ml of this solution with 900ml of 'fresh' DI, as this is a 1 in 10 dilution, it will give you 4dkh.
I wouldn't go smaller than 1L as you could risk making the solution innacurate,
DI water is available from the car section at Halfords,
Firstly, turn your drop checker upside down, and add 2 or 3 drops of bromothymol blue (the more reagent you add, the darker the colour)
Then squirt a small amount of dKH reference solution into your drop checker
Tilt it slightly so it runs into the ball, and pushes the air out (can take a little practice), hopefully you can see the air bubble escaping in this picture.
Turn the drop checker back over, and insert into the aquarium, making sure there is an air gap. Note the colour should be blue.
Try to fill the ball half full, as this ensures the maximum surface area for gaseous exchange, and therefore a quicker reading. Here is the colour recomended (lime green).
A blue colour indicates you have low CO2 (25ppm or less)
A green colour indicates your CO2 levels are ok (26 – 35ppm)
A yellow colour indicates the CO2levels are too high (36ppm or more)
Aim for a lighter/ lime green colour, as this ensures you have enough CO2.
Note: The colours in a drop checker take on average 2 hours to change colour, so be patient and adjust the levels slowly, as some people put the levels up high as they are worried they are not injecting enough, only to find they have gassed their fish.
E.g. A green drop checker at 3.00pm means that the level of CO2 was 30ppm at 1.00pm. So the levels may be slightly higher by this time.
Here is a picture kindly submitted by Supercoley1 showing the reaction times of 2 different types of drop checkers (tear drop left, chameleon right)
11am, fresh solution inside the drop checker.
3pm, CO2 starts
4pm, lights on with CO2 being injected for a total of 1 hour.
Start injecting the CO2 up to two hours before lights on, to make sure the levels have built up, and turn it off, up to 3 hours before lights off because the levels will remain high enough for plants, as they slow down their photosynthesis rates towards the end of the day.
Edited by aaronnorth, 06 March 2011 - 07:35 PM.