rozdaboff, the lens you have is perfectly fine for macro photography and it is more than capable of giving you the pictures you want. You don't need any other lens. Even if you want to go crazy worrying about chromatic aberration levels, it's easily removed in post, absolutely no need to get another lens when the lens you have is perfectly capable. All lenses "become softer" in the higher f-stops, its a result of diffraction.
The issue with your pictures isn't depth of field, or chromatic aberration, or lens softness, it focus. Those are all just unfocused (more on this later),...at f/18 you have plenty of depth of field and it's actually overkill.
As a side step, I just want to mention that the lighting is good, but its a little too harsh in my eyes, in other words, the flash aspect is just a bit too strong. Like in the shrimp pictures for instance, notice the harsh highlights and intense shadows?
There's a few ways to fix this, either use a slower shutter speed (which in your case, don't do, 1/60 is the lowest I would dare to go), increase ISO, or lower your f-stop....all of which will increase the level of ambient light mixing with the flash giving you more natural looking pics. The first thing I would do is maybe up the shutter speed to 1/100 (or up to 1/200 or more is probably better if you can do so with your flash/camera combo, but you probably won't be able to go past 1/200 since your not using a newer Nikon flash with high speed sync), but decrease the f-stop to f/11 or f/13 at most, and f/8 at the minimum. All the fish pictures in my sig were shot no higher than f/13, with most at f/8 to f/11, and thats on a full frame camera, so it'll likely be even more acceptable on a DX(crop sensor) camera if thats what your using (not sure what you have). The last way to increase the ambient and make the flash appear less harsh is to up the ISO. You don't have to go crazy, but even ISO 600 to 800 would help tremendously. But it depends how well your camera does at higher ISO, and for you, lowering your f-stop instead will be much more beneficial as you can still use ISO 100.
But anyways, besides the lighting, if you want sharp pictures, you'll need to change your focusing habits. It would help if you can tell me what focusing mode you're using and what camera you have, but I usually use AF-S for Nikon (One shot for Canon) with the single center point (you can use AF with 9 point expansion if you want), just make sure you avoid any of the auto settings, they're total crap. You have to be super fast in pressing the shutter after focusing, all the while being careful not to jerk the camera as you do so. You can use AF-C for Nikon (Servo for Canon) with the single center point if you want, and you'll also get good results, but I always had more success with AF-S despite using the same technique with each focusing mode.