Clean-up crews are meaningless in freshwater and brackish aquaria, and only mean something in reef tanks. You don’t need them in ordinary (non-reef) marine tanks anymore than in freshwater tanks.
The idea in a reef tank is that you cannot easily disassemble the corals and living rock, and if a bit of food or a dead fish gets stuck somewhere awkward, it helps to have scavengers that quickly break it down into something the filter, protein skimmer and water changes can remove. Likewise, you cannot easily use a siphon to suck up detritus because you risk remove small inverts and damaging corals in the process.
In a tank without corals and living rock, you can usually easily move rocks and wood to remove corpses, and use a siphon to clean the sand or gravel. Plants aren’t damaged by siphoning. Thus, you don’t need a clean up crew because it is always better to remove unwanted things directly. At the very least, by removing a dead fish or a bit of food directly, you avoid problems of adding nitrates to the water or anaerobic decay in the sand.
Also misunderstood is the effect scavengers have on a tank. They increase nitrates and phosphates, and in doing so, promote the growth of algae. Even a pleco, a so-called “algae eater”, will eat some algae, and may well clean the glass, but in the process will increase the rate of algal growth and elevate the level of pollution in the tank.
This isn’t to say you shouldn’t have scavengers or algae eaters in an aquarium, but you should recognize that you are adding them for your amusement, and not to serve a practical purpose. In a brackish water aquarium there are a variety of fish that work well in this niche. Mollies and florida flagfish work nicely as algae eaters, and there are at least 3 species of genuinely brackish water pleco. Violet and Awaous gobies are very effective eartheaters, and will remove small food items from the sand. Orange chromides, like other cichlids, are remarkable generalists, and usually eat a variety of small food items other fish miss.