Within the past few months, I have been having a huge algae problem in my large aquarium. After some research, I found that one of the many causes of this is too much light. Well, after cleaning out the tank, I tried leaving the light off for a few weeks, and it has been helping a lot. I also found out that leaving the light on will cause algae to start in the first place. The current light on it is an all glass f15t12 preheat fixture running a GE cool white lamp. Today I was looking on landscape lighting websites (learning more about iwaski moon pulse lamps) when I remembered that they are used because their light does not affect plants. Since the tank looks gloomy and sad without the light on, I was wondering if using a low wattage clear merc like an EYE 50 or 75 watt wouldn't encourage algae growth. I want to build a new hood for it soon (the current one is mismatched and looks rather ugly) and was considering using a small reflector for the lamp in it. I tried researching it on aquarium forums and they had mercury vapor confused with high pressure sodium (they all said it made an intense orange light) so it was useless. The aquarium store where I get lots of supplies has 175w tubular MH fixtures above the display tanks, and one had a 175w clear merc. The tank actually looked much nicer than the MH lit tanks did (it looked natural like a deep lake). So my question is, would it hurt to use a clear merc to light this aquarium and would it help with the algae?

I myself have tried this before and wouldn't recommend it, clear mercury looks completely different under water than how our eyes perceive the light color normally. It looks EXTREMELY yellow underwater for some reason. I used to keep African cichlids in a 90 gallon, and the tank came with a light hood that had two 175w metal halide fixtures built in as the tank was previously used to grow coral (saltwater). I didnt initially use the MH as it was unnecessary for my application, but eventually ended up using just one of the 175w MH fixtures centered in the hood with a blue 20,000K lamp, the tank looked incredible with the MH compared to fluorescent. I then thought a clear 175w merc would give a different look, and it was horrible. The tank was a greenish yellow mustard color now, which is interesting because the light looks blue-green above water. Needless to say I went back to the blue MH. Btw the "mercury" lamp you likely saw at the fish store was probably a nearing-EOL blue metal halide.

I myself have tried this before and wouldn't recommend it, clear mercury looks completely different under water than how our eyes perceive the light color normally. It looks EXTREMELY yellow underwater for some reason. I used to keep African cichlids in a 90 gallon, and the tank came with a light hood that had two 175w metal halide fixtures built in as the tank was previously used to grow coral (saltwater). I didnt initially use the MH as it was unnecessary for my application, but eventually ended up using just one of the 175w MH fixtures centered in the hood with a blue 20,000K lamp, the tank looked incredible with the MH compared to fluorescent. I then thought a clear 175w merc would give a different look, and it was horrible. The tank was a greenish yellow mustard color now, which is interesting because the light looks blue-green above water. Needless to say I went back to the blue MH. Btw the "mercury" lamp you likely saw at the fish store was probably a nearing-EOL blue metal halide.

I am pretty sure the lamp I saw at the fish store was a merc because it had an ED28 envelope, which I have never seen on an aquarium lamp. (the area is normally covered but they had it all taken apart and I sort of got a good look at the fixture. I couldn't see the arctube though since it was far away and the area was blocked off. It might have been an EOL metal halide like you said. I wonder why clear mercs would look horrible underwater? I would like to use a 175w 20000k (I have seen them and they look amazing) but the tank is too small for that much light and these lamps aren't made in lower wattages.

Try finding a lamp without the wavelenghs needed for photosynthesis.
An aquarium MH could be good.
A grow light (I have one, 7500K, And it's compact. But it's 400 watts) could work, And maybe have filters for no algae growth.

The trick here is minimal red wavelength, thats why Gro-Lux lamps are so pink and red-dominant, plants (algae included) require both red and blue energy for photosynthesis, blue doesn't seem to be utilized nearly as much as red however. Thats why HPS grows plants so well. The blue MH and blue fluorescents produce almost no algae growth since they're lacking red almost entirely. Also Blue metal halide can be had in nearly every wattage.

Heres a 70w HQI type (double ended) Blue (20,000K) lamp. All you'd need is a cheap 500w halogen floodlight and a 70w ballast and you'd be set
http://www.ebay.com/itm/70W-HQI-Double-Ended-20-000K-Metal-Halide-Bulb-20000K-70-Watt-T6-RX7S-Base-/251682913916?hash=item3a99787e7c:g:y3UAAOSwPe1T0GD0

They do make aquarium MH in ED28 as well as ED37, I have a Venture 10,000K 250w aquarium lamp in ED28

The trick here is minimal red wavelength, thats why Gro-Lux lamps are so pink and red-dominant, plants (algae included) require both red and blue energy for photosynthesis, blue doesn't seem to be utilized nearly as much as red however. Thats why HPS grows plants so well. The blue MH and blue fluorescents produce almost no algae growth since they're lacking red almost entirely. Also Blue metal halide can be had in nearly every wattage.

Heres a 70w HQI type (double ended) Blue (20,000K) lamp. All you'd need is a cheap 500w halogen floodlight and a 70w ballast and you'd be set
http://www.ebay.com/itm/70W-HQI-Double-Ended-20-000K-Metal-Halide-Bulb-20000K-70-Watt-T6-RX7S-Base-/251682913916?hash=item3a99787e7c:g:y3UAAOSwPe1T0GD0

They do make aquarium MH in ED28 as well as ED37, I have a Venture 10,000K 250w aquarium lamp in ED28

I have both a 70w ballast and a cheap 500w floodlight, but I was told that those MH lamps might not fit in them. I was considering that though.

70w and the lesser common 100w DE R7s type lamps like the one in the link will fit perfectly into the lampholders of a 300-500w linear halogen (Ive tried it). They are exactly the same length and same base on each end. The 150w versions will not fit though as they are 1/2" longer than the halogens. But 70w and 100w will definitely work. Be sure the 70w ballast is also PSMH and not HPS, as it will not fire up on a simple HPS reactor.

70w and the lesser common 100w DE R7s type lamps like the one in the link will fit perfectly into the lampholders of a 300-500w linear halogen (Ive tried it). They are exactly the same length and same base on each end. The 150w versions will not fit though as they are 1/2" longer than the halogens. But 70w and 100w will definitely work. Be sure the 70w ballast is also PSMH and not HPS, as it will not fire up on a simple HPS reactor.

Alright, thanks for your help. I am sure it is a PSMH ballast as it came out of a 70w PSMH canopy light that was damaged in a tornado. (the building maintenance gave it to me).

Do 10000k bulbs have any red light in them? I don't think an intense blue MH would look good with my tank plus I have seen 10000k's in use and they still looked nice.

Do 10000k bulbs have any red light in them? I don't think an intense blue MH would look good with my tank plus I have seen 10000k's in use and they still looked nice.

To get 10,000K, You would need something to offset the blue, So you would add red and green, Or it would be 20,000K.
So I think it has some red.

Zarlog, Most 10,000K lamps are just ever so slightly bluer than 6500K, My Venture 10,000K might as well be 6500K because its nearly identical in color to my EYE Iwasaki 6500K 150w lamp. Also, nearly all 10,000K lamps are a rare-earth fill containing Dysprosium and therefore will produce a more complete spectrum than an indium only lamp like 20,000K. I also have a few generic chinese 10,000K lamps and the color between them is quite variable. Some run whiter and some more blue. But I think the general consensus is that you should expect nearly all 10,000K lamps to basically look like a 6500K daylight. My Plantmax 250w 7200K is actually Bluer than any of my 10,000K lamps. If you want something in between the daylight white of 10,000K and the deep blue of 20,000K, have a look at the 14,000K lamps, I have a Hamilton Technology 175w 14,000K lamp and its color is really nice, not too blue, but not too white.   

Zarlog, Most 10,000K lamps are just ever so slightly bluer than 6500K, My Venture 10,000K might as well be 6500K because its nearly identical in color to my EYE Iwasaki 6500K 150w lamp. Also, nearly all 10,000K lamps are a rare-earth fill containing Dysprosium and therefore will produce a more complete spectrum than an indium only lamp like 20,000K. I also have a few generic chinese 10,000K lamps and the color between them is quite variable. Some run whiter and some more blue. But I think the general consensus is that you should expect nearly all 10,000K lamps to basically look like a 6500K daylight. My Plantmax 250w 7200K is actually Bluer than any of my 10,000K lamps. If you want something in between the daylight white of 10,000K and the deep blue of 20,000K, have a look at the 14,000K lamps, I have a Hamilton Technology 175w 14,000K lamp and its color is really nice, not too blue, but not too white.   

Alright thanks. I have never seen a 14000k in person but I have seen pictures of them online and they look like they would also be quite nice.