I work as an Aquaponics Engineer for a large firm. Some background on the aquaculture. I have had a population of 25 tilapia for 4 years. Reared from juveniles, I had not lost a single one until two weeks ago. They were recently moved from my research system at a local college, to one of my four main system tanks here in PA. A two weeks ago we lost our first of this population. We did think anything of it until we lost 5 more. They were healthily established in the system for two months. The death of the first fish followed the addition of 800 juvenile fish transferred from our nursery system. I did some autopsies and they appear normal on the inside. Their gills have great color, and their livers seem fine. I was leaning away from any water quality problems as the problem seemed limited to the population in that tank. Also, all levels of unionized ammonia, nitrites, and nitrates are well within the safe zones. I had them quarantined for precautions. The 800 juveniles began to die off here and there as they approached 50-60 grams. As you can see in the pictures, the larger fish developed sores on the mouths and lowers jaws. They do not seem to have bugle eye, or redness around the anus. Their blood and gills also have great color. The smaller fish have been dying off at about three per week. They initially did not show any sores, but we were typically decayed by the time we would find them. With both the adult, and juvenile fish, we noticed that the population in each tank would go through phases of not eating for up to 24 hours at a time. They would then feed fine for a day or so, and then stop eating again. These times of not eating followed the discovery of new fish deaths. We have not noticed any piping or top breathing from the fish.

Photos: https://www.dropbox.com/sc/m4hoehzpwf8wreh/J3tbrjBgu_

https://www.dropbox.com/sc/m4hoehzpwf8wreh/J3tbrjBgu_
I did tests on the algae and found four different species, including U. zonata, Gonium spp, and Euglena. No toxic species were present, and all species were in low enough quantities that I did not see any of them as a culprit. As you can see in the pictures, most of the juveniles exhibited a green tint around their pectoral fish. Also, a small hole was present between the two fins, this led to evisceration. Some of the recent juveniles have shown small, orange/pink speckling on the lower jaw. This too is noticeable in the photos. On a side note, when we started to notice sickness in the juveniles, I moved 50 fish to a research facility. They were placed in a tank that shares water with another tank that houses healthy tilapia from another population. This was to see if the disease seems to spread by water, or simply by contact. All of the fish, in both tanks appear fine, with no deaths in either tank. We were looking at sources of the disease, and are thinking it may have come from the fish that we purchased from our supplier. The first round of 800 fish seemed fine, but it was when they were introduced to the main system that we started to see problems with the adult fish, and later with those fish. In the newest batch that we got (800) there were six fish that appeared to be dead for more than 72 hours. 29 fish were DOA, and within the hours of me having them, 69 more fish died. I know I have given you a lot of information, but I figured the more background the better.

I set up a hospital tank with heavily salted water. The adult fish were hospitalized and we have not lost any since. Moreover, I disconnected the main system tanks (juveniles) from the plant growing equipment, and salted their water as well. I have done a couple of water changes, maintaining high salinity, After a few days of salting the fish, I did a large system water change, approximately 60 percent, and have reconnected the plant equipment. We have lost a couple of fish during this process, by the general population seems to be feeding well and is acting normal.

Thank you so much for your help on this. Any guidance you can give would be greatly appreciated. Feel free to pass this on to anyone you think would be able to help us out.