Photo period when switching to MH
 

Just received my new lights today and have them on my tank now. They are 48" MH 2x250W with moon lights. I had a 4x24W T5HO on it from my old 55gal while I was waiting. They are on a standard 90gal now. Just wondering how long I should have them on to start while my corals adjust, its a lot more intense light. My poor toad stool leather shrivelled up with in the first half hour.

tagging along.

I would either go to Home Depot and buy some window screen material and put a few sheets over the tank to "shade" the light a bit. Then every week take a sheet off so that the light is getting more intense by the week.

Another option is to run your lights 1/2 hour on, one hour off, 1/2 hour on, one hour off and then gradually "meld" the photoperiod into one long photoperiod over the course of a few weeks. (This is how I did it when switching from 150 watt halides to 250 watts) It just takes more fooling around with the timer. I was too lazy to go buy screen material and rig it so that it would "sit" over the tank and under the lights.

Hope this helps.

+1 Cut your hours back and increase slowly.
Watch for corals going lighter in color move them to the side or down a bit, and raise your lights if you can.

Thanks for the replies. I think I will try starting at 4hrs/day and slowly increasing from there. I hope the heat won't be too much of an issue otherwise I may just sell this and go for a 48" T5HO light fixture. I do like the shimmer though:)

4 hours at once? That might be a bit much, keep an eye on your corals for bleaching.

I bumped it back a bit to be safe. Does one hour on, one hour off, one on sound okay? I have a torch coral, open brain, gsp, xenia, toad stool, and a BTA. They are all on or near the sand bed at the moment.

I would stick to 1/2 hour on 1 hour off just to be safe because your corals really didn't have too much light before compared to your new light/new bulbs. I know it's a pain, but I'd like to error more on the safe side.

I was told that turning mh off and on frequently like that is hard on the bulbs as well as the ballasts. Someone correct me if i'm wrong please. I would have them turn on for just a couple of hours where you know it will be the middle of your lighting period, increase the time by 20 more minutes each day until you reach your actual time they will be on every day. 10 minutes earlier and 10 minutes later each day. Most people on run mh's 6-8 hours per day. Also think that it will stress your fish out if the lights keep going off and on. Everyday when my mh's turn on there seems to be a burst of energy with the fish for a couple of seconds (freak out). And yes heat will be a bit of a concern during summer months.

I'd try and get your lights higher up for a while. If that's not possible then the previously mentioned layers of screen should work in a pinch.If you raise them, drop them an inch or so every week until they are where you want to leave them.

Dropping an hour from your photoperiod may not be a bad idea either. However I certainly would not switch them on and off. Remember you are trying to simulate a slice of ocean in your house and in the wild, the sun does not pop up and down every few hours. Will it hurt the fish ? Probably not but I don't think it's good for the lights/bulbs.