Congratulations! Common musks are lovely little guys to keep.
Firstly - a suitable Musk turtle home.
Musks are part of a group of turtles loosly classed as 'bottom walkers', that is to say that they don't so much swim as walk allong the bottom. As a result they are actually not the greatest swimmers, particularly when hatchlings. Baby musks will be a lot happier in relatively shallow enclosures (floor area is much more important than volume). The best things that I have found for raising this kind of turtle (Muds, Musks, etc. etc.) are plastic under bed storage boxs.
They measure about 32"x15" and are 5 or 6" high. With 4" of water these are perfect for hatchling muds and musks, and are pretty cheap too. Your 18x18 is fine, but similarly should not have more than four or so inches of water for them to be at their happiest (were you to fill it to the top they would exhaust themselves constantly having to struggle to the surface).
Gravel should be removed for a number of reasons - it presents a choking/impaction hazard, it makes cleaning a pain (more on this later) and in trying to burrow into it they may cause themselves damage if there are any sharp edges.
A basking area should be offered but will be rarely used (they just don't bask as much as the larger turtle familys), more important are things like hollow plastic logs/shelters to climb on/under. Bunches of oxygenating plants such as elodea and watercress (cheap bags from the supermarket, well rinsed) will provide cover, food and help improve water quality, so don't stint - but they will get destroyed so go for cheap ones and don't bother aquascaping, just let them float.
UV lighting must be provided. However heating the water is completely unnecesary - a total waste of time, these guys are hardy, their natural range in the wild extends as far as canada!
As for filtration/water treatment you really have to get into a totally seperate mindset from fishkeeping. Cycling is totally pointless unless you wish to keep fish with them (actually possible, I'll elaborate in a sec.), and unless your tap water is particularly nasty water treatments are also pretty optional (I'll use a little dechlor, but thats about it).
Musks actually enjoy a little muck at the bottom of their tank (if you look closely at their chins you'll see two little barbels for rooting around at the bottom), so its not critical to keep the tank spotless. By far the easiest method is to use a (small) cheap mechanical filter purely to remove waste material (in fishkeeping terms vastly under filtering) and simply wash it out in the tap every couple of days. with plenty of plants and a reasonable diet this should be adequate to keep the tank clean and healthy (with water changes every couple of weeks, straight from the tap).
This sounds HORRIFIC to a fishkeeper, but years of experience have taught me that the result is clear water, no smell and a small layer of muck at the bottom of the tank - perfect for Musks!
As I said above, Common musks are one of the few turtle species you can safely keep some fish with. Simply choose hardy, small and above all fast fish and the musks won't be able to catch them (generally, I mean I've lost one or two over the years so don't go for anything too expensive!). I've used Zebra danios, white cloud minnows and harlequin rasbora over the years quite successfully, and they help clean up some of the crap! Obviously if you intend to do this you will need a fish safe setup with correct filtering and so forth.
Well that was a bit of a mini essay! Hope it helps.
Alex.
