Okay so is this normal?

[jjmorton13]

Side mounted under tank heaters are close to useless when it comes to increasing ambient temperature or substrate temp. You should be fine with the heater under the tank. You can always put something under the tank corners to lift it off the table a bit.

[Kevin1]

Like others have said, It sounds like a heat problem. a light bulb wouldn't be a bad idea either.

[jelkins]

Place the heat-mat under your enclosure...They work well and I've experienced no ill-effects on my African Bullfrogs.

[Bruce]

I have a heat pad under my 40 gal long that I keep on during the winter months (gets pretty cold in my room) and, combined with heat lamps, works great! It can dry out the substrate pretty quick though, so keep an eye on that