Sounds like a nice seahorse tank.
IMO I would add about 30 lbs more of sand and just call it a DSB. What you are at now is going to give you just under 3" which is kinda of a danger zone for sandbeds. IMO you need to aim for 2" or lower or 4" or more.
As far as lighting, in a tank that tall and that narow I'd probably lean toward a MH bulb myself. The problem here is the bulbs made for the nano setups are like 70w's or something no? Guess it depends on what your looking at. That wattage is not going to give you that much penetration deep in your tank.
Personally I use a 150w HQI bulb on one of my reef tanks that seems to burn pretty cool. I only have 1 2" exhaust fan but there is no temp difference created by the bulb when it turns on. It is a reef tank so the temp is onstantly 79F kept that high by a heater (my house is in the 60's most times). I also can't tell you if that fixture will penetrate the 27" to reach the bottom of your sandbed.
I can tell you that I had a tank that was just as tall and ran a dual 65w PC fixture and was able to grow mushrooms, zoo's, leathers, and macro's on the sand bed. If you want to go with PC's you could get a couple of 96w quad PC bulbs. They are about 18.8" long so would still fit in your canopy.
The difference in heat output between a good MH and a couple of 96w PC's is probably going o be pretty comparable IMO (just an opinion never measured the difference with a thermometer, . . . although i could, I do have both running
)
How's that for decisive.
I don't think that the MH's will be to bright for seahorses. Seahorses are often found in very shallow waters. The brightness from your MH's is not going to compare with the brightness of the sun in htat shallow of water IMO. They seem to do fine in nature.
On the temp issue, lower temperatures are used to help control bacteria mainly, not parasites. While I'm all for having a clean tank and a good cleanup crew it will not help the control of the bacteria.
The majority of seahorses carry bacteria internally so it is not something that can be kept out of a system. This bacteria is called vibrio, and there are several different strains that seahorses carry. These bacteria's are managable and non aggresive at lower temperatures. At higher temps they become far more virtulent and begin to excreet entirely different proteins. The bacteria at these temps not only gorws and reproduces significantly faster but is alos much more likely to cause disease. In the ocean there is the dillution factor, in ou tanks, not really practical. Labratory test have shwon that the bacteria's most likely to cause diseases in seahorses growth rate and aggresiveness is substantially less at temps of 74F or below.
Many keepers choose to keep there tanks at higher temps. There are many cases where this has shown to be just fine. Personally I choose to go for lower temps because IMO it is safer for the seahorse and makes my life easier. Treating a sick fish sucks. I've had pretty good luck doing this, not many people around have kept WC's as long as me.
If you want to read more about the temp disease thing there is a book with a chapter written by Dr. Belli called, "Working Notes". There are also chapters from Marc Lamont, Pete Giwonja, Clare Driscoll, about various other seahorse disease related topics. It's a good book and something I would suggest you pick up.
Good Luck and HTH
I'm the same way - too short to reach the bottom of my aquarium.
