sand sifting star fish mating
 

I have many things breeding and spawning inside my reef tank. Three days ago, my sand sifting star fish started mating, and they are still at it !

What exactly do you mean by "mating"? Starfish don't really mate, they spawn - males release sperm and females release eggs, but it doesn't necessarily mean they have any physical contact. Is your tank clouding up? If so, get some water ready and do a big water change -size depending on your tank's volume. If you have a small tank do a huge waterchange, if you have a very large tank you can do a smaller waterchange. Fresh carbon will also help to absorb organic waste.

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One star fish sitting on top of another one. I did a search on google, and that's what I came up with.

The coolest thing I saw was when my hermit crab spawning.

Sand sifting stars are pseudocopulating, external fertilizers. They "mount" each other to get their gonads close together and increase the chances of fertilization, but they still release their gametes into the water to meet up

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should i break them up? and toss one inside the sump? I don't want them to sour the tank.

I've seen them spawn before, but not on top of one another. Interesting. I'd split them up.