Macro is very difficult to get into (well get really good at haha). Best thing to do is increase your DOF by increasing your f/. try some shots around f/10, drop your ISO as low as you can (i usually shoot at around 100 and rarely 200). for best results color wise, place your flash on top of your tank, the further forward you put your flash, the more isolated from the background (which will appear black) your subject will become. Using an overhead flash will isolate colors more instead of wash them out (what usually happens if you use direct)
are you using a tripod? the soft focus makes me want to ask. Also while the result is very nice, most people will complain that the one is too dark thus lacking definition.
The next two have excellent light, could be brighter but i prefer a bit darker. You could get a greater depth of field if thats the look you're going for.
In shooting coral like that i would be tempted to use a reflected overhead flash (take a Styrofoam box and place it on top of the tank and point the flash up into it, same with the fish. This will disperse the light and create an excellent even lit area when shooting. It will also utilize the Tank lighting for even more color.
The fish pics i would increase your DOF again, they move quick and it'll really help get a proper focus on the whole thing
again a little dark for my taste.
Edit: One more thing, it really helps to clean your glass before a shoot. Do it half a day before and that will give the dust time to settle. (you're getting a little noise just from dirty glass)
judging by your kit you probably know most of that but I thought i could suggest them for others as well
Otherwise Awesome first shots