start by assembling the base to look like the picture. The corners should fit like the second picture.

Next, glue the top of the base on. I made it out of 1,5mm wood. As well as the corner posts. I left these a bit bellow the base to add height. 

Glue the piece with the rocks on next. The corners will fit perfectly.

repeat the assembly from step 1 for the top piece. Flip the lid over so that the middle piece is closer to the top. Then glue it in the same way as the base.

Add the details next. This will finish the tank.

the ornaments are double sided so if you glue them back to back it will be double sided.

Next glue the fish on where ever you want.

Then put the ornaments in and your done.