Surprised at the strange aggressive tone that some of these responses are taking. It's awesome that at 14 you'll be getting into this hobby. Creativity is key as is the ability to provide the right conditions for thriving animals.
There's a lot you can do in a 20 long. Without having to add rock. My suggestion is to look into macro algae-scaping. This is a wonderful part of the hobby that is often over looked. But a marine macro tank can rival any freshwater planted tank. Not only are tons of varieties, but they provide cover and reduce nitrates/phosphates.
Also, gives you the ability to add more substrate. Which opens up the door to some very cool invertebrates, e.g., Bumble Bee Snails, Nassarius Snails, even a single fighting conch. Porcelain crabs are awesome. They don't need much space. Fun to feed and watch.
With the extra plants, you will make some room for extra bio load. But you need to test.
For fish, look at the Neon Blue Goby. Super tiny and can be paired up to show that behavior. With added texture in the water from the plants, your clowns will have fun exploring and picking through the algae.
You also can add some hermits, which I would suggest if you do this, being that they are great at rummaging for hidden food, etc.
Actually, for extra fun, depending on the size of your clowns you could look into an Angler. The behavior would be fun to watch.... Although you pretty much nix any other smaller nano fish, and even some slightly larger fish. More, discerning ambush predators are the pygmy scorpions -- they won't just stuff anything into their mouths. Then you can house smaller mollies or ghost shrink and watch that life cycle take place.
Honestly, if you setup a cool marine planted aquarium you probably won't tear it down. You'll likely just keep both the 40 and the 20. You can also just combine the systems and make an awesome display refugium with actual life.
Man, even some feather worms, with dragons breath. And some fern calurpa.
For dosing, you just need very little manganese/iron supplements and you can do this manually...a few drops a day.
But of course, without quarantining fish you risk a lot. I know Mr.
@Humblefish always quarantines invertebrates but the risk is lower. Just slowly drip acclimate and transfer a couple of times from a couple of buckets to reduce the risk of bringing in any water from the LFS or online order. I use a bucket to dump them in, drip into it for like 45min. Then move them one by one to another bucket with my own tank water already in it -- - which is the same as the water they've been drip acclimating into --- note that you should remove water in the drip bucket periodically until you feel the old water has been replaced. Then drop them into the tanks.
