You know it dosen't leak so the next thing to do is look carefully at your seams inside and out. You are looking for any imperfections. From the outside there should be no cracks or scalloped glass and no apparent fail of the silicone seal between the panels (no air or dirt having intruded into this space). From the inside again your looking for any apparent imperfections in the silicone such as significant lifting off the the glass, dirt etc under the silicone, or missing seal altogether where it has been ripped away. Gently rub your finger on the seal, it shouldn't lift from the glass. If all looks well then don't touch it. If you find a specific area damaged you can overseal it with silicone patch. If you find it is rough all over then you'd have to cut away the interior seal (not cut apart the panels) cleaning all traces of silicone off the glass and running new beads around all corners. If the joints themselves are in rough shape you are probably best off to take it to a tank repair place and have it taken apart and re-siliconed.