Legal question:Selling house with built in tank, are fish pets?

[TheMikey]

I'm not a real estate lawyer but:

What does your sales document say? All fixtures generally go along with a property transfer and any additional terms or conditions are written in (i.e. all appliances, microwave, washer/dryer, etc.).

At the end of the day, what does the contract/agreement you've executed say? If you put your signature to it, you've promised to uphold your end. This is a contract dispute, so a determination of your obligations will go back to what was written and what was said by each parties (although the written part is much more concrete).

Does the sale contract include the aquarium livestock? Did the purchasers ever say "we want to keep it all?"

If not, you indicated first that they wanted it all gone and had no interest in the tank. This is why you sold your livestock and (presumably) also tried to sell the hardware?

Sounds like you might want to go to a different lawyer.