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2.
Term used for those who cruise in a convertible with the top down and the windows rolled up. A car in this configuration appears like an aquarium or fish tank, especially after a good rain storm.
Check that geek in the Miata, he must be fishtankin it to keep his wig on.
by Skizix June 21, 2004
 
1.
The act of two people making out with such fervor that it is audible to others around.
Example 1:
(Josh and Lana are making out)
Marlee: Can you hear that?
Amanda: Is that the fish tank?

Example 2:
(Alexander and Kayla were making out the previous night)
Alexander: (sly look) So, I kept hearing a belt buckle last night...
Nathan: Yeah well, I heard you fishtankin' it up! How about that?
Alexander: (holds head down in shame)
Nathan: (ultimate glory)
by curlyfries102 January 30, 2011