How To Lower Alkalinity In Fish Tank. Reverse osmosis filters force water through a semipermeable membrane that traps and removes solids from the water. Alkalinity much below 50 mg/l is a very dangerous situation and could lead to ph crash, aka “old tank syndrome” (see a related article in this section of the web page).

Be sure to prepare the water and test its ph before adding it. Alkalinity much below 50 mg/l is a very dangerous situation and could lead to ph crash, aka “old tank syndrome” (see a related article in this section of the web page). To keep your fish happy, you have to maintain an adequate water ph in your fish tank as the ph will measure the alkalinity and acidity of the water in the aquarium.