CichlidFish.com - Fish That Think.

my Oscars are fine all day but at night when i turn off the light they start to fight each other. why is that?

Don't know -- usually the opposite happens! Bright light brings out the aggressiveness, turning out the light calms them. Is one fish dominating the other? If they're wrestling around and they seem to be on even terms, it may be "mating play". If one of them is dominating and hurting the other, you need to get or make a tank seperator and keep them apart --

--Mark

how big is you're tank sdothogg? lack of territory could be causing issues as well. You should have a minimum of 75 gal (in my opinion) with a nice wide footprint.

yep, only masters can manage a small tank (55 gal) with two Os, even then they need a bit of luck to have Os that like each other.

Another thing that is possible, but should present itself not just at dark, is some toxicity somewhere.  Might have some nitrate creeping up?

If you do have a smaller tank, you could try removing decor, reducing the depth of the substrate (more gallons=more swimming space and more water for waste dilution).

Have no idea though  ...  you could have a 900 gallon tank with 1 ppm nitrates, if so ... 

But some common information that could help other help you better would be:

Tank size, filtration, all the tank-mates, approximate size & age of fish, decoration saturation (sounds nice to me  ) and type & thickness of substrate, tank backing if any (sometimes letting them see out of the back makes them feel more secure, as if they have more room/territory, your space is their space even if they cannot access it), temperature,, pH, approx. ppm for ammonia, nitrite, nitrate, diet (feeders might breed aggressive behavior), where the tank is located (noise, traffic, air vents, stoves, fridges (lights on and off), dishwashers, etc.) ...  anything you can think of really.

If you have them in a big enough tank then you probably have two males. I am surprised they are not fighting in the day time as well. Maybe they are and you are not seeing it. You can get dividers for tanks. This would stop one from killing the other. Redfeather

I had my first O in a 10 gall... dumb of me...  but i got a 75 gal tank 2 months later when he/she was 3 in, but i didnt no i would get such a big tank so soon so i thought that i could try gettin another O that didnt work atall, he/she ( i think it was a he cuz of all the fighiting) he anialated the second O
i woke up one morning and cloud (oscars name) was looking very very satisfied with himself... then i saw the oscar...dead in the corner . i wuz stupid

i have a 75gal tank

The easiest way with the least amount of guessing is to net your Os and take them to you local fish store to be vented (call first to make sure there is someone there who knows how). That will tell you if your problem is the sex of your Os. Also take a water sample with you. Most fish stores will test it for you for a minimal fee. If territory is the issue, make sure you have at least one large piece of either wood or stone approx. in the middle of the tank to divide the space. If you have decorations there now there may be too much, not enough or they may be too small to determine separate space for your Os. Good luck.