Cleanup crew refers to any creature that cleans and eats things like algae and other inverts. The most common clean up crew in a tank are small hermit crabs, various types of snails, sea urchins, other types of inverts like bristle worms, starfish and bristle stars, and some kinds of fish.
The cleanup crew helps to get rid of pest algae in the display tank. By doing this, it makes your tank look a lot cleaner without a bunch of algae covering eberything.
There are creatures in the clean up crew that con be so good at getting rid of algae, that even with the worst water chemistry, as long as they are healthy and there is enough, it can leave your tank sparkling clean. (except for the glass, most things don't really go up to the glass and eat algae off of it)
Here is a more detailed list of of the creatures in typical tank clean up crews
A lot of marketers (espetially of hermit crabs) will say that you need 30 hermits per gallon. This is not true, as they will fight to the death for food, and you will usually end up with 1 or 2 for every 10 or so gallons, depending on how much you feed the tank, what is in it, and how many ricks are in there. So if a marketer offers you a "value pack" of cleanup crew, dont get it unless you have a huge tank. It may seem convincing to get one, where theya re usually like 10 bucks for 50, and a single one is like 3 dollars, it is better to spend a few more bucks on making sure you have a hood tank. Also, if you get the value pack and have many dead hermit crabs littering your tank, they wil decompose, producing an algae bloom that your hermits won't be able to fight off
For the snails, it is a but harder to say a good amound on how many snails to get. Im my experience, I have a 30 gal, and after a few months, there were only 2 or 3 snails left, and they died soon after. If I were you, I'd get one per every 5 to 10 gallons, ans see what will happen with them, as every tank is different.
Moving on to shrimp. Shrimp not only eat algae, but cleaner shrimp will also set up a cleaning station in the wild, where fish will stop by, and the shrimp will eat things like dead skin and parasites off of the fish, keeping them clean and pest free. Since shrinp are faster than, say, a snail or hermit crab, (and larger) you will need less, like one per 15 gallons or so, depending on how many fish you have. Also, Peppermint shrinp will eat common pests like Aptasia anemonies, which can sting and bother corals, so they will be good to have as well. One to two for every 50 gallons should be good. Also also, pistol shrimp will burrow in the sand and under the rocks, and will come out at night to forage and clean the sand, so they can be good if you have a problem ijn the sandbed. (the pistol shtimp also have one large clay, and can make a loud snap if they feel threatened, which can stun nearfy fish and even break glass if the shrimp is big enough, so be careful when you get one. However, shrimp are more sensitive to water conditions, so a shrimp or two while you are starting out should be ok. But if they die, Its better to let the tank get more established before you srated again.
Urchins do a very good job of cleaning the algae off of the rocks. They will eat any kind of algae, and Ive had one clean the entire rockwork covered in hair algae in just a few days. They are so aggressive in eating algae that they will even eat the beneficial coraline aglae. This seems bad, sinve coraline algae doesn't bother corals, and keeps other algae from growing on the ricks, but the urchin can make a lap around your entire rockwork in a day (Smaller sixed rocks), so any algae that does grow will get mowed away before it becomes an eye sore. Since urching are such good eaters, you don't really need a few in a tank. Depending on your algae problems and water chemistry, you can add one for every 50 gallons or so.
Emerald crabs. They are one of the few cheap and hardy predators of bubble algae, so if you have a problem with that, you should definately get one. They are small green crabs, though I have seen one tha twas bright purple. They will not only eat bubble algae, but are also a common predator of many stony corals like duncans. They will eigher straight up eat the polyp, or will pry open its mouth to get the food within.
Tangs are quite large fish, and anything under 30 gallons is NOT big enough to house a tang. Between 30 and 75 it is possible to raise a tang, but it is likely that it will be malnourished, or may die fron stress caused by not ehough water to swim in. There are many stunning tanks, and a simple yellow tang might not eb pretty enough for a beginningn aquarist. They might lean towards a purple tang\, like Dory from Finding Nemo, but this is NOT recommended. Though they are hards, they do need lots fo space ti live. They have huge territories in the wild, and like to swin hundreds of miles every day. Unless you have a 100 gallon pklus tank, DO NOT get a blue tang, or any other larger tangs for that matter.
Sand sifting gobies are small, usually white fish that sift through the sand, hunting for algae and other smaller creatures to eat in the sand. They will dig tunnels if they can, and will stir up the sandbed, and may cause sandstorms or make piles of sand on one corner of the tank. However, they are great sandbed cleaners. They can clean the sand of up to a 50 gallon tank overnight, even when theya re an inch long. One on its own can even keep the sandbed of a 100 gallon tank relatively clean. Therefore, it is recommended to get one for every 50 to 75 gallons of tank water. but this depends on what kind of tank you have, is it a 100 gallon frag tank that is long and shallow eith a lot of room for sand, ir is it a same zixed cube, where there is much less room for sand?