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The bulb in the light over my 65 gal. tank went out last week. This tank is decorated with just bogwood and anubias, has 2 fluval 3Plus filters, and is home to a big common pleco, 8 SAE, and 12 young-adult male Rainbows of various types.
My LFS doesn't carry compact fluorescent tubes, so I have to wait for them to come by mail order, so the light has been out for 4 days already. The day after the light went out I was trying to see how things were doing in that tank, and I noticed a dead fish wedged into a crevice of wood. It was my large Madagacar Rainbow, and as luck would have it, my favorite fish in that tank. I had one large madagascar, and 3 smaller ones. A day later, I was looking and saw another dead fish stuck to a filter. It was pretty torn up and all the scales were gone so I honestly am not sure what type of rainbow it was. This tank has been running for 14 months now, and only a single SAE has died in that time, so I was very surprized that two fish had dropped in as many days with no other problems going on in the tank aside from the light going out. Over the weekend, I tried to take a count of the remaining fish and account for everyone, but they are all active fish and difficult to count. One thing I didn't see, however, was ANY of my madagascar rainbows. Not a one. It is not unusual for dead fish to be picked clean and devoured by the other fish in that tank and scavenged by the dwarf crays I also have in that tank, often before I even realize I've lost a fish.

So, how is it possible that the lack of light over that tank has impacted fish health? And only to one species of rainbowfish? It seems very strange to me? What do you guys think?

Hoping that there must be a word for everything I mean...

Rainbow fish, including the not so closely related madagascan rainbows are not really vulnerable to photoshock as most fish are, infact the radical differences in illumination often stimulate them into courtship.

They are however VERY vulnerable to low oxygenation, and adverse water quality. They require regular water changes whether nitrate readings are low or not, and long term keepers of this species will vouch for this. It is impossible to keep them in entirely static water bodies without regular changes.sometimes 20 percent changes may need to be done as often as twice a week. The compound in aged water that kills them has not as yet been identified. It is not something you can read off a test kit.

Periods of darkness in planted tanks can cause the plants to use o2 instead of co2, thusly reducing the dissolved oxygen in the water, and i suspect it is this factor combined with a bacterial or protozoan infection that is killing your rainbows.

It's also much easier for fish to see out when their tank is dark and the room is light. Sometimes quick movements or lots of motion around the tank can upset the fish when normally it wouldn't. If the tank is brighter than the room then the fish can't really see out and aren't as easily stressed. Try looking out of your lit room at night compared to without the lights on. It might help to cover the front of the tank in the evening when the room lights are on and the tank is not lit up. You could also pick up a cheap light at a hardware store or places like walmart until your new bulbs come. You can get 2-4' single and double bulb fixtures for $5-$10. While it wouldn't be a great setup it would help all the problems you might be having from lack of light.

Ah... good explanation longhairedgit. Thanks.

Well, my new compact fluorescent tube arrived yesterday, I installed it into the fixture, turned it on... and nothing happened. Apparently it's the fixture itself that burned out and not necessarily the bulb. It's a 36" Satalite fixture that takes a single 96 watt tube. Needless to say, I am irritated. I have a spare regular fluorescent fixture hanging around, but that also has only one tube, and since it's not compact fluorescent, the total wattage is under 30 watts, I think. I've had that fixture for only one year, so long enough for the warranty to expire, but not long enough to get my money's worth out of it.

Hoping that there must be a word for everything I mean...