I used to work in a theater in town as the lighting director. I also assisted with sound-checks, so I was standing pretty close to some really large monitor speakers for a pretty good stretch of my workday. After about 2 months, I developed tinnitus (which is that annoying ringing you get after a loud noise, just one that never leaves...). I also lost most of my low-range hearing. The only way I can "hear" bass anymore is to have it loud enough to actually feel the vibrations, and my ears still ring. I left the theater 4 years ago (due to other occupational hazards, i.e. near electrocution), and the problem still is there, and always will be according to my doctor. If I had only known then, I would've worn ear protection every day, because there is so much I miss now.
As far as determining a safe level for your listening pleasure and for keeping a fish tank nearby...
The most obvious thing to do is to turn the volume up where you normally do, and observe what happens to the room. If anything vibrates, rattles, shakes, etc. or if your ears hurt or ring during or afterward, or if you can't hear the telephone or someone has to shout to get your attention, it's too loud, both for your tank and for your ears.
By the way, I'm only 23.... and permanently damaged from 6 months of working in a loud environment, and only 3 or 4 hours at a time. 30% hearing left in one ear, 68% in the other. It doesn't take much.