This isn't a good set of questions in all reality. Saltwater doesn't work like much of fresh in terms of being compatible together and cleaning.
How big a tank depends on the fish as many can be open water swimmers that need at least 6 ft long tanks to be happy, like tangs, even though the fish can be the size of your hand.
Cleaning isn't much on livestock and more on you feeding and doing water changes along with any filtration you might run on the system. We do use hermit crabs and snails to do some work, but nothing will replace your time and effort.
In terms of plants, you could mean algae or coral. This is a very broad scale that can mean a simple vho
bulb to very intense lighting from metal halide or strong led
This all effects cost, which then is determined by the route you go. A good example is a led
panel to grow anemone or sps
corals can range from 100 - 800 dollars!
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