To my knowledge there are 2 good magnifying glasses for the tank. first is the one form flipper that @
BZos106 posted. The second is
the portal. I currently have the flipper brand and it works great. The magnet is a bit weak for my 1/2" glass and it sometimes falls off when I move it too quickly (it doesn't float so I have to fish it out). The price point is spot on for what it is.
I also owned the portal and iirc it has 8 magnets that couple the magnifier to the glass, I've never had it separate. The price is a bit more (I would imagine the 8 magnets drive the price up) but it can also be used as a glass cleaner as well (so can the flipper but it has less surface area and will take a long time to clean a big tank). I owned the portal for over 8 years and one of the magnets on the wet side got wet and I can no longer keep it attached to the tank, I occasionally bring it out when there's multiple people looking at the tank.
In the terms of magnification, they both are comparable, with the portal slightly winning out. however when trying to view something further in the tank (say 12" back) the flipper has better clarity and focus.
My biggest fear was having the flipper fall to the floor and break, but it has proven to hold on tight enough that I don't worry anymore.
Personally, aesthetically I like the portal, but in the end I would have to choose functionality over form simply based on price.