I just got a used aquarium from my cousin. It's dry, but there is black stuff on the sides, and I'm not sure how to clean it off. It's completely dissassembled, but the parts are inside. Everything except the gravel is open. I can get a picture if needed.
To clean an used aquarium you need to get some kitchen sponges similar to the ones here https://www.amazon.com/Sponges-Kitchen-Cleaning-Tools/s?ie=UTF8&page=1&rh=n%3A15754811%2Cp_n_location_browse-bin%3A3530187011
Be sure they do not contain any soap or detergents,Be sure to use only clean water when cleaning inside the tank.
You need to empty the tank of all of the equipment and gravel before you start cleaning it,Moistening the algae and dirt makes it easier to get off.
The pump and filter needs to be cleaned too,You need to put it in a bucket with water to moisten the dirt and remember to clean it both innside and outside,Use q tips for the hard to get to spots.
If the filter sponges is in good shape you need to clean them by rinsing them and squezing out the dirt,If you are in doubt just replace them.
The glass on the heater needs to be cleaned too,Use a kitchen sponge for this be careful it is made of glass.
To clean the gravel/sand use a strainer and fill it half full with gravel and rinse it in clean water,It will take some time but it needs to be done,Even if the gravel is brand new you need to rinse it to remove the dust.
When all is nice and clean place the tank where you want it and install every thing,Fill with water and add declorinator start the pump/filter and heater.
It is now time to cycle your tank take a look here: https://www.fishlore.com/NitrogenCycle.htm it is very important to do this before you add any fish,Plants can be added as soon as the water is heated to the temparature you want in your tank.