You are using the wrong med. What was the dosage used? Quick Cure is composed of malachite green and formalin, not just formalin. Malachite green is very harsh to small fish such as tetras. Dose formalin at half strength (one drop per gallon). May I ask what tetras do you have? Increase temperature at 80 degrees Fahrenheit and switch on all airpumps as formalin will deplete oxygen levels.