I want to build an aquarium light fixture using this type of lamp. I have a 300/500w halogen floodlight, and was wondering if the lamp would fit in there? It is a perfect fit for the fixture I want to build, but I don't know if the lamp will fit. Will it?

The base called RX7s and not RX6s, and I think that it is possible to install 70W and 150W RX7s MH lamp in a R7s halogen socket for 300-500W lamps. You need a remote ballast and ignitor to operate the lamp.

As far as I know, only the 70W fit into the 300/500W lanterns. The 150W are longer...
And even with the 70W you may have problems with the width: Some sockets are too small, so the socket pushes the pins side wise. And that is not good - the cement holding the contact on the lamp tend to break.

And worth mentioning that some wires used for the halogen holders never expected 5KV from a HID ignitor

Also the cement tends to fail from presence of moisture, which may well be the case above an aquarium

It should be possible to replace the lamp holder with a proper MH RX7 for a minimal amount of money. Then you'd be sure the intended lamp will fit. I am considering a conversion as well and that is what I'm planning to do.

It should be possible to replace the lamp holder with a proper MH RX7 for a minimal amount of money. Then you'd be sure the intended lamp will fit. I am considering a conversion as well and that is what I'm planning to do.

That's what I thought too. The major problem? The cheapest I can find some holders for is $15:00   

That's what I would expect to pay for one here.

The holder itself is ok unless it is damaged, but sure slip a few layers of glass braid isolation tube on the wires coming to it

Speaking of which, definitely get some of this tube to have on hand. It is useful for heat and mechanical protection of wiring whenever you repair a HID or high power incandescent lantern

And worth mentioning that some wires used for the halogen holders never expected 5KV from a HID ignitor

Also the cement tends to fail from presence of moisture, which may well be the case above an aquarium

Any insulation for 230V installation should withstand 4kVrms for at least one minute. That is about 6kV peak. So a 3.5kV ignitor would be perfectly safe from that perspective.

With the mechanical dimensions: Up to now I've always managed to adjust the socket within it's tolerances so, the lamp did fit properly. I never needed more than about 0.5mm, while the socket was possible to readjust by about 1.5mm...

I seen some 300W uplighters (they were common here at some point as indirect room lights) where the wire isolation is equivalent to a porous material. Just some straight threads in with the Cu conductors and on top of that the glass braid, like a paracord. That mean ~1mm air gap between conductor and lantern metal where the isolated wire rest on the metal

Won't those lamps get so hot they will cook your poor fish!

Won't those lamps get so hot they will cook your poor fish!

Not necessairly. These light fixtures have a cooling fan which keeps the inside of the light from overheating, and any reflected heat gets cooled off by the cold water. My aquarium is colder that it is supposed to be, even with a heater, so this will probably help somewhat.

Better fan not the lights (they dont need it) but the entire canopy over the tank. Also this allows you to use bigger and slower fan, so less noise and less dust clogging (which will eventually make the fan fail)

For aquarium light fixtures-a lens or other barrier should isolate the lamps from the contents of the tank-that way no danger of water contacting hot bulbs causing sudden failure-or if a lamp blows-the fragments don't get into the tank.The lens also serves as a heat barrier so the fish won't get cooked.The blower can then cool the lights and no danger of dust being blown into the tank from the blower cooling the lights.Also grow lights should have the lens so a blown bulb won't discharge the debris onto edible crops being grown.