Your tap water will be fine for your fish, do check the nitrate reading though.
Don't do a 75% change as the tap water is likely different to what is in the tank. I would do a 25% change every 2-3 days for 2 weeks and then get into a routine of larger changes once a week.
I haven't commented on your gouramis issues because I do not have gouramis and don't have much experience with fish illnesses but this is not likely to be due to nitrate
Don't do a 75% change as the tap water is likely different to what is in the tank. I would do a 25% change every 2-3 days for 2 weeks and then get into a routine of larger changes once a week.
I haven't commented on your gouramis issues because I do not have gouramis and don't have much experience with fish illnesses but this is not likely to be due to nitrate