If the tank has a flange around the top, so that the flange is parallel to the lid of the tank, I recommend liquid silicone...
Clean the flange and the inside of the tank lid.
Lightly sand the flange so that it's actually clean and a bit rough.
Apply car wax to the inside of the tank lid, let dry and buff.
(Do this at least twice.)
Squirt a bead of silicone around the flange, then spread it out so that it's wide and flat, about 1/4" thick and almost as wide as the flange. Spread it as evenly as possible when you do.
Allow it to dry enough, so that it skims over and doesn't stick to your finger when you lightly touch it. (Usually takes a couple of hours, depending upon ambient temperature).
Place the tank lid on the tank and weight it down with as many bricks as necessary to get an even pressure on the lid. (6 bricks on a typical rectangular tank) Of course, you can use anything that has similar weight.
Let it set at least over night. (It can require as much as two days in colder weather, in my experience.)
Take the bricks off and "pop" the lid off. (It will slightly stick and need to be pried off.) Check it, and if it still feels pliable, weigh it down again and try after another 12 hours or so.
Usually, it's good to go after an overnight period to set. This is especially true if it is in a warm ambient temperature.
This type of seal is a bit involved to put on, but lasts for YEARS...