Night and day overflow/return rate change

[Airgoose2222]

So this is very odd to me. I have a 300 gallon tank with duel corner overflows. It's been up and running for a couple weeks(ish). The overflow and return rates haven't been an issue until I started using my metal halides. There are two 400 watt bulbs running. Since I've started using them, the flow rate changes when I turn them on and when I turn them off. In the morning, once they go on, I have to adjust the overflows to decrease the flow. At night when I turn them off, I have to increase the flow or the sump gets too low. After adjustment, they're fine all night and all day. It's just when the lights go on and when they turn off. The metal halides sit a foot above the tank and are about two and a half feet from the corner overflows. Anybody ever heard of this before???

 

[Scooterdragon]

Are your lights and pump on the same power circuit

 

[Airgoose2222]

Now that never dawned on me. Yes they are. As are the sump and overflow on my smaller tank which is doing the same thing. Thank you for that! Didn't think the power drawn from those lights would make a difference. I'll try running them on a different circuit tomorrow and see what happens. Excellent deduction!!

 

[Airgoose2222]

Plugged into a different circuit now. Much better!! Thank you for that advice!