I don't trust to drop checkers. Too slow reaction and not so correct, sometimes i think my eyes are wrong, lol.( I found info in internet - Japan people see much more colors and tonalities as White Europeans !) This piece is nice, but need check ph minimum 2 times every day. Drop checker can show green but ph can be much lower.
And if will be some electrical shortcut or lights go off - overdosing, disease - fish can die,algae, ...
I bought digital ph meter for self in Ebay for 23 GBP, highly recommended.
Plese look chart standard co2 test solution what use all drop checkers - bromothymol blue
green on PH 6.4 can be too low and can be too late - overdosing, i keep min. 6.6ph and in morning before co2 on is 6.8
Situations is different, possibly 6.4 can be fine, depend from many factors kh,gh, temperature,..
My info just for reference, i hope this info helps avoid some mistakes
I tried once shut off at 6.3ph co2 and switched on air pump with open top of aquarium and for raising in 44gal tank ph from 6.3ph to 6.6ph needed half day
Very carefully with all co2 things
Also almost all ph tests is wrong +-1,2 due different temperatures or have color scales like 6.4:6.8:7.2 - potencial overdosing or too low level
I highly recommended digital ph meter with ATC(Automatic temperature control), once calibrated shows very exact ph longtime +-0.05
Fish Health is more important, and i don't understand why need extreme fast growth if later plants throw away
Re: DIY Drop Checker 
Linear Mode
