My Guppies Keep Getting sick, but with different things

Thank you, I've finally decided on a plan, today I put the carbon back in my filter and did a 50% water change, scrubbed the algae off rearranged some things and noticed how empty this 40l tank looks with 8 guppies and 3 tetras, so I'm gonna put my two bigger fry in (both females) to replace the two that I lost, and when the others are big enough I'll put the males in and enough females to keep them all happy then give any remaining ones away, if the females do have more fry then I'll just l3t nature take its course and rely on natural selection, I might add some more neons (healthy ones this time haha)

Sounds good! I'm glad every things going well and hope it continues to!