HELP! losing fish!!!!

[ggee1]

Having some problems here..... have a school of medium sized jacks which we cleaned the algae off the sides of the tank and moved around some gravel. Water was replaced with fresh sea water, next morning 1/2 the fish had died and the others looked pretty stressed with lesions on the sides. Did a 50% water change and this morning there were more dead, that looked fine after the h2o change the night before. I'm at a loss as to what has caused this with no prior signs. Any ideas would be apprecated!

[White Tiger]

FIRST OF ALL, STOP DOING ANY WATER CHANGES FOR NOW!!!!!!!!!

Doing a 50% water change can stress a fish out and possibily kill it if it is sick or already stressed out.

Do the surviving fish have any redness on their body?

Do the surviving fish seem to be breathing rapidly?

Do the surviving fish have raised scales?

Was the temperature of the new sea water significantly different from the temperature of the main tank?

Are there any white blotches that do not look like ick on any of the fish?

Do you always do a 50% water change?

[ggee1]

Water changes are not normally 50% however, the water was pretty bad with the large number of fish that had died.... and was the only thing that could be done at the time to maybe say the ones that were left.

Breathing was labored, prior to the water change and yes there is some blotching on a those few fish no redness though.

Temp of the sea water was one degree warmer then the tank...

[BTTRFLYGRL]

A 50% water change should not kill your fish..As long as the temp, ph and SG matched ...I have done more than 50% when my fish were in qt
What did you use to clean the algae?
Where fish dying BEFORE the water change?
Other symptoms?
How long have you had the fish?
How long has your tank been set up?
Size tank?
Run test on the water and post them...need a little more info

[White Tiger]

Where the fish breathing rapidly before any of the fish died?

Where there any white blotches before any of the fish died?

[Steven Pro]

Is it possible there was something wrong with the seawater, like a red tide event or something?

[ggee1]

The tank has been set up for about a year now and these jacks were raised from small hatchlings caught off the coast here. The tank holds around 1000 gallons. These fish have been doing great for about 8 months, no disease, and up until the tank was cleaned they were active and feeding...

All I can think of is that there was something in the substrate that became toxic. I was able to save two permits after removing them and putting them into quarantine. It's a mystery!

[White Tiger]

"All I can think of is that there was something in the substrate that became toxic." (ggee1, 2006)

Could be a possibility. Earlier you stated that your fish had white blotches on their bodies. White blotches may indicate an irritant is present in the water.