Used tank water will already have nitrates in it..
Not the best plan and possibly why you are having issues with water..
Very little good bacteria live in the water column . They colonize surfaces not the water..
old tank water is garbage and exactly what you want to get out of tank..
Possibly doing large change on big tank with plenty of new water will dilute the nutrients enough to make that water better then what the fry are in but I would try if you feel comfortable to give fry new water..They will need this throught your raising them..
I breed and raise a lot of fish ,but I change more water..Can't do the one without the other..Poor water quality is 90% of why fry fail often.
I'll add using a conditioner like Prime can detoxify the nitrite for 48 hours. Follow the directions and know the nitrite will still show on test and is only detoxified for 48 hours.
If in 48 hours the filter does not process the nitrite into nitrate you will need to dose again ,which should be done with a water change..