Very Rapid Guppy Deterioration Two Tanks

[star4]

There maybe a chance then that the work on the water pipes has caused the problem, even if your water was not cut off to do the work.   With them flashing as if they had ich but not signs of ich I would suggest adding some activated carbon to the filters see if this helps at all, just in case there is an irritant in the tapwater. I know if its bacterial carbon wont help, but it might be worth trying just in case.

 

[missparadox]

Water co said that there have been no changes. The 90l had been getting salt added in and the 180l had very little done to it bar a few water changes. This morning they looked the same and I lost another one. However tonight exactly 72 hours later they all in both tanks perked back up again. There is only one or two in each tank that are looking a bit off. The others you wouldn't think anything had been wrong. 
 
To me as TwoTankAmin suggested it was nitrite poisoning. Would explain a lot of it. But I can't understand how it would happen at the exact time on two completely separate tanks. Tanks that are on separate WC cycles, use different conditioners with nothing between them shared. 
 
I am leaving in a few weeks time for a trip to America. I will be gone a month and would like to know what it was in case it happens whilst I am gone so that I can give a heads up to the house sitter. 
 
In any case, thank you so much guys! You all did a lot in putting me at ease.