Hyposalinity treatment affecting bacteria cycle in QT?

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Aquarium Advice FINatic
Aug 12, 2009
Cincinnati, OH
Hello. I want to run something by the experts here, mostly for a sanity check. TL;DR, two tangs died suddenly in my quarantine tank and I suspect it's because the hyposalinity treatment I did affected the cycle-bacteria in the tank, causing ammonia poisoning. I'm wondering if this is possible/likely and what I can do for future QTs.

Now, for all of the information.

The main tank is a FOWLR tank, 150 gallons + 30 gallons sump/refugium, started in 2009. Many fish have come and gone over the years but due to issues with my own health and global health the past few years I haven't added in any fish in the past 4-5 years. There are only two fish left, a damsel who has been there since 2009 and a 5yo mandarin. I want to restock the tank so recently I started out by getting two tangs.

I have a 20g tall quarantine tank that I set up, I did a fishless cycle over the course of two months and before adding anything to it, I was able to put 4 ppm of ammonia in the tank and have zero ammonia and nitrite readings after 24 hours consistently (I did this every day for a week). The morning I got the new fish I did a 100% water change on the quarantine, then I added the two tangs from the fish store (with an eggcrate divider in between. The tangs were both small, maybe 3in. long, one clown tang and one naso tang.

For 2.5 weeks everything went extremely well. Once the nervousness went away, both fish were eating very well and very active, showing no signs of disease or parasites. Over the first week I gradually decreased the salinity from 1.025 down to 1.009 as a preventative treatment -- in 13 years I've never once bought a tang that didn't have ich so I just did a preventative treatment, expecting to hold them at 1.009 for 4 weeks without symptoms, before bringing them back and putting them in the main tank.

Then one day, very suddenly, one of the fish died. The other fish was fine that day but the next day the other one died as well. I removed both fish immediately after they died. That evening I did tests on the water and there was a small but nonzero ammonia reading, with zero nitrites. I was confused. At the lower salinity I tried putting 4 ppm of ammonia in the tank and it took 2 days to get back to zero ammonia and nitrite after that. I can't really think of anything that could have happened to cause this issue with the bacteria in the system other than doing the hyposalinity treatment.

Since then I haven't added any fish to the QT, and I don't really want to add more fish to the QT until I know that I'm going to be able to give them the best chance possible at surviving. This fish store I'm going to now seems to do things really well, and these tangs were really healthy when I got them. The obvious thing is to just try again and not do hyposalinity unless I see signs of ich, but I'd at least like to do my due diligence and find out if there's anything else I should be looking at that could have caused this.

Right now the QT is at 1.025, and I'm just increasing the amount of ammonia I can add while getting back to zero ammonia and zero nitrite within 24 hours. I'm at 5ppm at the moment.

Any thoughts are welcome, I can give more information if needed. Thanks.
Im no SW person, but 5ppm ammonia is too much. At that level it could kill the bacteria you are trying to grow. Water change to get it down to 2ppm and then wait for that to cycle out. When that's cycled out add 2ppm, wait for that to cycle out. When you are dosing 2ppm and seeing zero ammonia and nitrite 24 hours later you are cycled.

Dont keep adding ammonia if it isnt dropping.
At this point, for about 4 days now, I've added enough ammonia to get 5ppm, and within 24 hours it's going back to zero (both ammonia an nitrite). I guess I don't need to increase it any further.
At this point, for about 4 days now, I've added enough ammonia to get 5ppm, and within 24 hours it's going back to zero (both ammonia an nitrite). I guess I don't need to increase it any further.
Correct, I would not add any higher amounts of ammonia. Once it cycles out in 24 hours or less, you would only need to add small amounts of ammonia to keep the bed alive until you added fish. The bacteria bed would shrink some ( compared to the 5 ppm) but when you add the new fish, it would grow rather quickly to accommodate the new bioload should it exceed the level of ammonia you have been adding.

Tangs and ick have a long history of being connected but in reality, the reason why most tangs got ick was because they were stressed out either by being in too small of a tank or mishandling before they were purchased. With today's knowledge about best Tang husbandry, I would not do any " preventative" theories of the past unless you actually see the parasite. A freshwater dip just prior to adding to the main tank should suffice. ( back in the late 1990s, We typically did this to all the incoming fish due to flukes they were coming in on almost all Pacific Ocean fishes. :( )

You did not mention which species of tangs you purchased but if you split a 20 gallon high, you severely reduced the swimming space for these fish. I doubt that was the cause of these 2 fish's deaths but could have been a factor that reduced their ability to handle other stresses. In the future, with tangs, most round species are best put in last to your collection and most elongated ones do not fight amongst other fish and other elongated species so no need to separate the tank for 2. If you have the option of doing a 20 long vs the 20 high, that would be better, especially for tangs since they are swimmers. (y)
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