PLEASE HELP I THINK MY PIRAHANAS ARE DYING :'(?

[shell]

DATE - 3rd Feb

ok so long story short i bought this tank with 2 red bellies in it, when i was speaking to the guy i thought he sounded like he knew what he was talking about and knew his stuff about tanks etc so anyway when i got the tank in the water just about covered the fish so i proceeded to fill it to just over the filter like he said and both fish were hiding which i figured was from shock/fear etc from being moved and having hardly any water for however long anyway this morning i was watching them for a bit as u do lol and both fish have found a place each in the tank and are just sat on the bottom doing that opening and closing mouth thing like gasping like fish tend to do with lack of oxygen at the top of the water (if that makes sense) and i thought these fish really dont look too great so joined a cpl of fish groups on facebook to see if anyone could give me any advice and thats where i see the same person asking about other types of fish coz he is getting a new set up with sharks or something and someone said about cycling his new tank and he didnt have a clue what they meant.... so thats my first Question....... Do you need to do the cycling process for pirahanas??

he also said there is no way he would have an empty tank for the length of time it takes to cycle a tank properly which tells me that this tank defo hasnt been cycled. he gave me "a bag of food" which was some prawns from the supermarket, and thats when i thought this really isnt looking great......

on top of this, the filter that is in the tank is throwing water back out that powerfully that the fish cant swim about, there is no lid on a tank with fish that jump also no lights and there is 2 of them which i know isnt good as there should be a min of 3 otherwise one will eat the other - am i right so far please???

anyway i went to the pet shop and got proper food, some tap safe and i bought a new tank (71 litres/ 2ft) with filter, heater etc and he even gave me some of his media and tank water in a bag to hurry along cycling.

so my next Question is where do i go from here how can i get the fish into the new tank so they can get better?? or what do u recommend i do now to stop these 2 fish from dying please,?

should i use the carbon out the old filter in the new filter with the new carbon to speed up the cycling process, im so confused ?

also i know i cant buy one new pirahana to make the numbers odd but what would happen if i was to buy 3 more would they be ok? or if i was to get one that wasnt still a baby as one of these is about 4 inch and one about 2 inch that we have already sorry for the length of my question thankyou

Thankyou in advance any help is appreciated asap pls