I'm going the opposite way.
I was FF Canon for years but several years ago I started having eye trouble and neuropathy issues due to diabetes. My balance isn't always the greatest and the heavy camera and lenses were killing me. I went mirrorless a few years ago and now have the Olympus EM1 Mark II and love it! Good glass is always going to be expensive but I have found the prices a bit more manageable with mirrorless. I have a lovely 45 mm prime 1.8 that was less than $500! The lenses and equipment are lighter for me and easier for me to handle. I have a pet photography business and I'm always on the move.
Invest in a good macro lens for your aquarium and I look forward to seeing your images!