It is zero risk to her job. Some people get angry when told there care is bad. If the boss doubts the care she researchs it. If the boss believes the care she follows it. If the boss ignore it then tell the person yourself.
In my experience, it's more dangerous to be passive agrressive than direct. When something is done passive aggressively, it gives off the image that you bottle things up until you explode and then act like you don't have a mushroom cloud above your head. If you think something is being done wrong it is more professional to tell someone in private to their face in a direct manor.
Also, tone is not conveyed well in type or gestures. Quite a few arguements have happened on the forum because people read the wrong tone in a message.
The same could happen with a secret pamplet since it could come off as judgemental instead of informative. If the OP just tells her boss face to face, the boss will know the OP isn't being judgemental but caring. The OP noticed a problem, and suggested corrections for the sole purpose of correcting an error. No judgement. No Holier than thou mentality. Only straight to the point facts.
In fact, if I was in the OP's postion and I noticed my department manager had a betta in that set up, I would say, "Excuse me, I've been caring for betta fish for a little over 9 years and couldn't help but notice your's. Bettas are tropical fish that need 78-80 degree water and at least 1 gallon of water to survive. A better set up is 2.5 gallons with a heater and thermometer because 1 gallon tanks are so unstable. Ammonia spikes happen out of no where...the smaller the heater the less reliable it...2.5 gallons is a nice mix of stability and small size."
However, I'm a bit blunt and would have spoken my mind the first time I saw her fish in too small of a set up. I have my two 5 gallon tanks as my screen saver on my computer and have talked about tanks with a co-worker during lunch. It isn't a secret that I'm obsessed with fish.
The OP may have to mention that she's been worried about insulting the boss, but wants to share some knowledge before suggesting the upgrade.