The plastic bracket?
Not sure which you mean. Is this the trim along the bottom of the tank?
Or does the tank actually rests on the plastic bracket, namely as a floating tank?
The only juwel 83 litre I can think of is like a Roma 90, I used to have one of these and that has a floating tank base which is the plastic bracket you probably speak of.
As Colin T already says, you can remove the plastic and have a polystyrene or rubber base to put the tank on.
Always safer to put a tank on a thin layer of poly or rubber base as any imperfections in the glass or any debris, even just a few grains of sand or gravel underneath the tank glass may cause the glass to crack under the immense pressure of water and substrate weight.
But there’s no real need to remove this plastic unless you really want to, just ensure you lay the tank on an even and clean base preferably with a layer underneath.