So I have been thinking of using my glass 125 for a reptile enclosure for about 6 months maybe then back to fish. Worried that maybe the silicon could dry out in that time. The reptile needs 85% humidity so would you think that may save the seals. If not I won't use the tank and go with plan B.
I've kept reptiles for a few years now. And there have been times when I quickly need a hospital tank for some fish, so I put my geckos in a different enclosure and used their tank. I've never had problems with the silicone sealing. And the gecko tank is kept in the 80-90 F, and low humidity, so even the dryness and high temp has ruined the silicone. Silicone usually degrades with age, so I would check the age of the tank (if the tank wasn't resealed). There's usually a slip of paper in the bottom edge of the tank. Just look at the bottom of the tank edge and you should find a piece of paper that has the date of when the tank was made.
And an acrylic tank will get scratched up really bad from the lizards. Especially if you're planning on keeping bearded dragons. they enjoy clawing at the walls. stick with the glass tank is my advice. you shouldn't have problems with the silicone that are the result of keeping it as a reptile aquarium.