Generally speaking (and I acknowledge that this is a generalisation) most of the marine environments we like to emulate are from very stable conditions. The sea forms a massive buffer to any changes, wheras our river and pond fish get rapid condition changes due to rain and other changes.
As a result marine tanks need to be a fair bit more stable than freshwater.
That said, there's nothing to stop you stripping the filter in that and setting up a tank with some inverts and small corals, marine nano's are certainly doable but they do require a careful eye and a fair bit of dedication. It's a good way into marine from the cost point of view, but it's not the easy path.
Beyond that, the question is, as pingu asks, what is the stocking plan? Kit for marine varies considerably depending on what you're planning.
Best comparison I can give is between a high tech planted tank and a species only predator tank in freshwater. The kit requirements are fairly different, so a set up question has to start there.