How correct would one be presung an SNR 'Selfie' holding the camera at arms length, maybe 3 ft, pointing backwards?
If so,
1/ the closest focus distance on a 50mm lens will be something over a foot.. I have long arms, I might be able to hold the camera two and a half foot from my face if I lea back a bit but its stll rather a short subject range!!!
2. The Depth of Field, front to back focus on a 50mm lens between the nearest focus range and perhaps 3 feet will be measured in mm at almost all apertures.. focus will be critical and limited to a very slim zone infront and behind the subject range, that s small enough to see noses and ears drop out of focus, if you focus on the eyes.
3/ With a 50mm lens at that sort of close range, I would not expect to be getting full length photo's... it would be a bit tight to get head and shoulders portraits.. whch hints you may not be dong arms length selfies... but still... a 50mm lens, is probably a tad too 'telephoto' for this sort of situation, where I would imagine that a kit 18-55, and probably set at the wider 18,, end towards perhaps the middle 35mm zoom setting would, probably be the more appropriate
Basic advice would be to back up, put more space between your subject (you & the fish), and the camera, probably a couple of meters or more, get some-one else or a tripod to hold the camera, if needs! (You may need to use the self timer or a remote release) Use less zoom, or a shorter lens. Check the camera settings, you probably don't need it setting the very low f-number apertures a 'prime' 50mm lens can, so pick an con mode like the head and shoulders, to get the camera to pick a more moderate aperture, or go semi-manual, using aperture priority... to pick one yourself, something higher than f4 I would suspect....
BUT also eliminate the 'dumb' and check that the lens IS turned 'on' to Auto-Focus! and you aren't knocking it off adjustment how you are holding the camera or something, and read the manual to see what focus 'scheme' its using... if you are using a single point AF mode and that dots not on you or the fish when you aren't looking through the view-finder, the focus could be anywhere!