Need help new to fish!

Hey guys I bought a white molly yesterday along with a red tail shark. However in a 3 hour time period whilst I was at work I came home to discover my red tail shark has dissapeared. I checked the floor and all surroundings I thoroughly checked the tank and tank items and he’s nowhere to be found however my Molly’s stomach looks like this and I don’t think it did once I first bought him can someone tell me what this is? Thanks guys also I have 1 shrimp in the tank with him and 12 neon tetras

It's very possible that he died and your fish ate him.
How big is your tank? Are you aware of the nitrogen cycle, and did you cycle your tank before getting your fish?

Did you check inside the filter?

Naughts said:

Did you check inside the filter?

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Yes

FishFinatic77 said:

It's very possible that he died and your fish ate him.
How big is your tank? Are you aware of the nitrogen cycle, and did you cycle your tank before getting your fish?

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It’s a 85litre tank whst do you mean nitrogen cycle? Yes I ran the water and pump for a week before getting the fish

TivvysFishes said:

It’s a 85litre tank whst do you mean nitrogen cycle? Yes I ran the water and pump for a week before getting the fish

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The tank is too small for a red tail shark, so if he is still alive you should return him.
Running the filter for a week does nothing. Your tank is not cycled. You have to do a 75% water change immediately.
When you have fish in a tank, they produce ammonia. Ammonia is toxic to fish, so you need to have bacteria that eat the ammonia and turn it into nitrites. Nitrites are also toxic, so you need to have a different bacteria that eat the nitrites and turns that into nitrates. You then remove the nitrates in weekly water changes.
You have two options at this point. You could either keep your fish and do a fish in cycle, which is very stressful for both you and the fish, and it is considered cruel by many fish keepers. Or, you could return the fish you have now and do a fishless cycle, which is a lot safer and less stressful for everyone.

What @FishFinatic77 said ^^^

@TivvysFishes , this is a really good video explaining the nitrogen cycle