Definitely try at least zero EV, but I always work in the negative. I haven't had much success with positive EV. The darker image will help create a mood, too.
ISO 200 should be alright. I wasn't ever concerned about the ISO. I often have to shoot my fish at ISO400, even. The noise is all the water debris in the background, and this second photo is a lot better about it, actually. Micro bubbles, sand particles, detritus, whatever. If you can take the time to get rid of those, it really helps the fish stand out off the background. The other issue is that coralline algae. If you were able to catch the fish without that in the background, that would be pretty "flippin' sweet, pretty much the best I know of." You could cheat a little in Photoshop and select everything except the fish, then use the levels toolbox (ctrl-L, your best friend in tank photos) to bring down the RGB channel.
Oh, and don't forget auto levels (shift-ctrl-L) first thing. It doesn't always work, and you'll know right away. Either your photo will look obviously better, or very clearly worse. Auto levels is accurate about 90% of the time, in which case it saves you a few steps. I always attempt auto levels before I try anything else.