During prolonged treatments, especially when the filter has not been isolated, it is important to test for ammonia and nitrite on a daily basis in case the filter has been affected. Using malachite green and formalin As you might expect both of these chemicals are affected by variations in water chemistry. Both can be 'bound out' of the system by high dissolved and particulate organics such as fish waste, detritus and algae. Formalin is more toxic in soft, acidic water and also removes oxygen from water so vigorous aeration most be applied. Each 5mg of formalin removes 1 mg of dissolved oxygen from the water.
Malachite green is also more toxic at low pH as well as high temperatures.
Do not inhale the fumes of formalin and always handle in a well-ventilated area. Both are potentially dangerous. Malachite is a respiratory poison as well as being a suspected carcinogen. Formalin is extremely irritation to gills and should not be used where there may be existing gill damage or where skin ulcers are present. To prevent any misunderstandings, formaldehyde is a colourless, highly toxic gas.