Dying Barra Advice required

[Rich]

Urgent Advice Required!
11 days ago I had the misfortune of discovering what no aeration to a tank overnight does to fish. After playing around with my pump I forgot to switch the power back on. The next morning 29 dead barra Av size 18cm. Up until then the fish had been thriving. They were purchased as 10cm fingerlings 6wks previously and were feeding and growing rapidly.
My set up consists of a 500L tank supporting 2 growbeds. The water parameters currently are ammonia 0, nitrites 0, nitrates 20-40, pH 7.4. Temp 28-30
9 Days ago I purchased another 35 barra. These were slightly smaller about 6-8cm long. These barra were not as flightly as the 1st batch. Since then I have lost 8 (3 today) and at least another 4 are exhibiting the same initial signs. The fish (normally not more than 4) rather than spending most of the time at the bottom of the tank tend to swim close to the surface. At feeding they nudge at the pellets but have no appetite. They then tend to swim more weakly, sometimes being pushed around by the current and end up on their side and then dying.
The oxygenation in the system is the same as the previous batch (with fish over double the size of these fish) with loads of bypass water from the 3750L pump, plus auto syphons every 20mins plus a small aerator running constantly.
I just don't know what is occuring. I have added another aerator this pm.
If oxygenation is a problem do fish recover? and should I see an improvement.
Any thoughts on what is going on. Some fish are behaving normally as the last batch.
Any advice would be great

[aussieap]

Gday Rich. If ur water is clean, can only be air. You have a reasonable fishload in a small setup. Would be worth checking pH and temp swings morning and night too. Also, remember the higher the temp, the lower the available dissolved oxygen in the water. Dasall I can help with.

[Outbackozzie]

Would salt help?

[Rich]

I took a couple of the sick barra to the barra farm. They felt that the water parameters and oxygenation to the system was adequate. Their only other suggestion was that the pellets may have gone off in the heat or were slightly too large. If they continue to die then I'm stuck with some infectious cause and may have to get a post mortem done by the DPI to isolate the cause. They have had no trouble with the rest of the fish in that batch. I really don't know what else to do at this stage another 3 dead this am and another 4 dying.

[Dufflight]

I'd add salt and look to make sure there are no dead fish hiding in the tank.

[Outbackozzie]

Definately salt to 3ppt, how were the fish transported and acclimatised? Any different from the last time?

[GaryD]

Some people report problems with sudden pH changes due to high inflows of rainwater into their system. Is this a possibility in your system?

I support the idea of adding salt......and with Barramundi you can be quite generous with the stuff.

[djs-sa]

Well have you had a rapid temp swing in the water?
Well that's what is the problem from my experience after ur other readings seem fine. crank up the temp and salt them up to 6ppt for 3-4 days whilst on small pellets, by the way what size and type food are u using?

[Rich]

An update and some answers.
Death toll 25, the last 10 seem OK at the moment.
The system is covered by laserlight roofing so no rainwater influx to disrupt things.
The pellets they are on are supplied by the barra farm. Now feeding approx 3mm pellets, B4 they were >5mm. Keeping them refridgerated now.
There are no temp fluctuations in the system. Up here in Townsville the ambient temp is av 25min, 33 max. Water temp only varies by a couple of degrees.
Yesterday I took 3 sick barra for a necropsy. On wet smear of the gills and scales some ectoparasites were found. They were Trichodina. There was not a Large burden so still awaiting histology results.

[nick]

i think i had a problem like this with my murray cod and the answer was that the fish had not beeen eating so I had to get them blood worms and then move them to mince and then to mince and pellets and then finally to pellets.
THis might be the problem if you are having unexplained deaths.

Nick