Fantastic looking wrasse!
Trying to give some constructive criticism on the photo itself...
I think your shutter speed was a little low for shooting hand held at the focal length you were using. That gives the image that slight blurriness or shakiness. A good rule of thumb for shooting hand held (without a tripod) is that you want your shutter speed to be faster than your focal length (on a 35 mm camera).
I’m sure you noticed your dog photo is much crisper.
Digital cameras have a conversion factor general between 1.25 and 2.0 (you can search for you camera and the word “conversion factor†to find this value pretty easily if you don’t already know it). In other words at 135 MM with a conversion factor of 2.0 you need a shutter speed of at least 1/270 (so use 1/320).
The shutter speed and f-stop (aperture size) work together to control the amount of light exposed to your sensor. General speaking (and I just started trying to do tank photos earlier this week), the amount of light coming from your tank is a lot less than the amount of light taking a photo on a sunny day outside. To increase your viable shutter speed you can set your f-stop to a lower number which will decrease the depth of field of your image. Another option is to increase the ISO setting on your camera which is like using faster film (meaning it doesn’t need as much light exposure to create an image). I would experiment a bunch and try out those settings.
In terms of composition that is much more a matter of opinion and comes down to how YOU want the image to flow and feel (imo). Generally I’ve found the rule of thirds to be a great starting point.
Sorry if this was a little too involved, just trying to help you out.
