Welcome aboard.
Hopefully you can help me understand the circle of confusion. I find it coufusing. And I need someone’s brain to pick about MF closeup lenses.
You need to be close to your subject. Or you can use a wide-angle telephoto to "zoom in" from afar. This method gives you a very shallow depth-of-range (any macro has a shallow DOR), but, requires a LOT of light, and between lens & subject no light pollution or anything on the tank glass.
Marine housings, also known as underwater camera bags, come in cheap & very expensive setups. I've seen nice results with the large ZipLock freezer bags, usually doubled-up for security, and an elastic band around the lens so that the plastic is taught and won't remove from the image quality too much.
Amazon & such sell "tourist" camera bags so you can take your favorite DSLR or mirror-less camera underwater with you when you go snorkeling on your vacation.
If someone has a "frag tank", as in, water less than 12" deep, you can use those floating round glass viewing panes - and shoot through that.
Practice, practice, practice! Especially on flowers, or an apple or orange, to get your technique pat. Every camera / lens has a different "sweet spot" for macro. Find yours, measure the distances, and see how you can apply that distance to your tank buddies.
Also consider using a mirror/stand if your fish is easily spooked, and shoot your mirror.
LAST TRICK!!! PROMISE!!
Use VIDEO, in 1080p 30 fps, then on your computer you can freeze a specific frame and use that frame as a picture. It will be 1920x1080 pixels.
Also if your sensor is 12 mega-pixels or higher, in video mode, perhaps only 1 in every 4 pixels is "used".
This means low-light and color range looks much better. 4 inputs condensed into 1 input.
A video frame is basically 2 mega-pixels, versus 12 mega-pixels in picture mode. But for looking "on the web" or even printing 4x6, it will look great !!!
So those *amazing* pics you see on SPS/LPS seller websites, they usually just shoot a video in manual focus mode, and in one run take all the "shots" they want. Then you pick & choose the frame that gives the best results. 2MPix is enough for a website.
Enjoy this last trick, brought to you for free by a seasoned professional.
TL;DR Shoot video, then use a computer to select a frame, and transform the frame in a JPG image.
