So could a goby and a clown be kept in the same tank?
Also, can I use the same equipment as freshwater? (Heaters, filters, etc.)
You can keep them together in a big enough tank but it also depends on what species you are talking about. There's a bunch of different anemonefish and gobies and the small anemonefish are relatively peaceful but some of the bigger species can get aggressive.
If you know what species you want I can tell you what size tank and whether or not they will get along together.
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Yes you can use the same equipment for fresh or salt water aquariums. You use the same filters, heaters, lights, plastic plants and test kits. The only things you need for a marine tank is a hydrometer (to measure the salt), and limestone rocks and a substrate that increases pH.
There are 3 main types of Hydrometers, floating glass, plastic chamber, refractometer.
The floating glass hydrometer is the cheapest and they float around the aquarium. They have a mark on them, which shows you hen the salt level is correct. The main draw back to this type of hydrometer is their shape and they regularly break because of it.
Plastic chamber hydrometer is a bit more expensive than the floating glass type but only a few dollars more. There are numbers on the outside of the chamber and you fill the plastic chamber with salt water and a needle points to the salt level. This type of hydrometer is my preferred choice.
A refractometer is the most accurate form of measuring salt but also much more expensive than the previous 2 types. You put a couple of drops of salt water on a glass lens and look through an eye piece. There is a scale in the eye piece that tells you what the salt level is.