Thinking some more about this and taking into account (a) your level of experience based on your comments, (b) a guess that this “aquarium computer” is probably not cheap, and (c) knowing how much back-EMF you can get from a solenoid, my suggestion would be to get an opto-isolated relay module. These are widely available from e.g. eBay etc.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local