Hi. For me, there are two main considerations - heat and splash.
The intense heat from MH - especially for a smaller tank like yours means you will have a heat problem if the fitting is closer than about 30 cm as the beam will gradually heat the water. Not a big deal if the room is cooler than the tank, but in Summer when you are at work and the house is locked up might result in very high water temperature in the tank. I might even go a bit higher as the depth of the tank is not so great and the light wont be attenuated much by the air above the tank anyways. Do you have a glass cover on top ?? If so, then if you keep it very clean, the above advice still applies .
I assume you have a light cover to protect the MH bulb from splash. Unlikely to be an issue at 30 cm height (though still recommended) , but if you want to be closer to the water surface, then it is mandatory.
Cheers
The intense heat from MH - especially for a smaller tank like yours means you will have a heat problem if the fitting is closer than about 30 cm as the beam will gradually heat the water. Not a big deal if the room is cooler than the tank, but in Summer when you are at work and the house is locked up might result in very high water temperature in the tank. I might even go a bit higher as the depth of the tank is not so great and the light wont be attenuated much by the air above the tank anyways. Do you have a glass cover on top ?? If so, then if you keep it very clean, the above advice still applies .
I assume you have a light cover to protect the MH bulb from splash. Unlikely to be an issue at 30 cm height (though still recommended) , but if you want to be closer to the water surface, then it is mandatory.
Cheers