For the last week about 2 hours after lights out in the tank both my protein skimmers go crazy and fill the cups up with water untill they are overflowing. During the daytime when the lights are on they are fine.

I checked the ph and that was 8.4 during the day and 8.1 after lights out.

Salinity at 1.025

Hardness 8.8

Magnesium 1350

Amonia, nitrate, nitrite undetectable

Top up water is fresh water with no addatives.

Any ideas would be appreciated

are you feeding or dosing anything just before lights out?

I am only feeding in the mornings.

Would the change in temperature maybe do it, from 27.5 down to around 26 at night?

I have a calcium reactor which is always on, maybe it could be something to do with that?

Nah, the change in temp or PH shouldn't have much effect. Do you have something like a kalk topoff that comes on at night?

calcium reactor should not be on at night as it will lower ph. at night you ph drops naturally so leaving the calcium reactor on will drop it even lower.

what skimmers are they?

during the day turn off your lights and see what happens to the skimmer.

Maybe the reduction in power can speed up teh skimmer pump and cause it to overflow.

Come on reef everyones CA reactor runs 24/7. Tell my who turns their Ca reator off at night.

I bet your CA reactor runs 24/7.

Mine goes off at night, or at least the Co2 goes off, pH was dropping below 7.8 at night so I made a few changes to make sure it never goes below 8.0. Luckily my reactor set up is relatively huge compared to the tank & it can easily maintain ca & alk in just 7 hours a day.

Come on reef everyones CA reactor runs 24/7. Tell my who turns their Ca reator off at night.

I bet your CA reactor runs 24/7.

I think you will find that calcium reactors should not be running at night as i outlined. I have never run my calcium reactor at night as it could cause harm to my fish, calcium reactor drops your ph and in the evening fish still produce co2 which drops ph.

My recommendation would be to turn off co2 at night by way of a solenoid valve,

here is a link on schuran reactors.

http://www.reefcentral.com/forums/showt ... genumber=1

Schuran suggests cutting off the CO2 at night to avoid excessivly low pH

Reef I have a soliniod on my reator so I will hook it up to a timer. Always learning with this hobby. I hope to see a change in my tank.

Will it only affect the fish? or corals also?

Low ph can cause problems for fish and coral.

To raise ph dose Kalk at night works a treat like reef said Ph will naturaly drop at night. Im not sure of the effect of running kalk and a Ca reactor at night? Would it offset the Ph drop? also lighting your sump at night will raise ph