WOW, WOW, WOW. Don't fill that thing up indoors yet!
The black plastic is part of the structure of the tank and aids in supporting the loads experienced by that water weight.
I'm not familiar with a 50 gallon tank's dimensions but if it is similar in dimension to the standard 55 gallon the lack of the bracing could seriously impair the tanks stability, especially if it is the center brace.
Many people de-rim thier tanks, for aesthetic purposes, but most would probably not recommend this for anything much larger than a 20 gallon. I'm afraid whoever sold you the tank was either misinformed or hard up for your dollars.
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