60l tank containing 4 junenile F8 puffers. Temp 80, not planted, biological filter, light on 12 hours a day
Yesterday I tested my 60l tank water before doing a water change, Ammonia 0, Nitrite 0 Nitrates 40 (this is because of my tap water) PH 8.2. Did a 30% water change, treating tap water with dechlorinator before replacing, and adding a small amount of marine salt to make an SP of 0.002 (slowly raising my levels from freshwater to eventually go to Brackish at 0.008)
This morning I came down to discover all 4 fish gasping at the top of the tank and immediately tested the water to discover nitrites off the scale of my API test kit. Straight away I did a 70% water change.
The fish were then fine, however this evening they started gasping again so did another 30% water change as nitrites were at 0.25 and have raised the pump to airate the tank and put an air stone in.
My tank was established for 1 year with goldfish, then these were removed and puffers introduced after 1 week as LFS said tank was mature and fully established. I have had my puffs for 3 weeks now and have moved them from the first original tank into a bigger one, as I discovered it was too small. On moving to a larger tank I did nothing to the filter or ornaments/plastic plants other than to put them in the new tank, used the old tank water mixed with new, but did change the gravel for sand. Yesterday I added a new filter to my tank that contains carbon so I can run it in conjunction with the old one to get it to colonise before removing the old one
Yesterday was the first time I'd added salt and I did put a new cave in the tank (aquarium safe)
Why has my tank got high nitrites yet not had an ammonia spike (test water weekly) is it something I did during the water change, was it the salt? What should I do other than daily 20% water changes until the nitrite levels drop?
Please help, I desperately don't want my little puffs to die.