My Guppys Keep Dying. Have No Idea Why!

[redshaw404]

Hi. I am hoping to receive some advice and hopefully e
get to the bottom of why my guppies are consecutively dying. We had 4 and now have 1 male remaining. Basically all looked well until one of them started to swim erratically, stop suddenly and then sink to the bottom hitting anything in its way. Then briefly shoot off again but then sink until eventually they stopped and died where they ended. Its very distressing as we have checked water levels and all says thats fine, we also have 5 neon tetras who all are fine. We have a 120L aquarium and all the filters and aerator are fine and in good working order. Could anybody tell me why this could be happening. I dont want to introduce anymore fish if I could potentially be killing them. The aquarium is in my living room.
Can anybody help?

 

[the_lock_man]

How long has the tank been set up?
Do you know what cycling is, is your filter cycled, how did you cycle it?
What are the water stats, in terms of ppm? (Your idea of "fine" might be different to mine )
How long have the guppies and neons been in the tank?