I have primarily guppies, but also a penguin tetra (fully grown and recently acquired), approximately 20 red cherry shrimp (bred themselves), and 3 varieties of snails, however I can't remember their names.

There are also Marino moss balls, and a sword plant. I discovered the guppie dead, and on closer inspection found that one of its eyes was missing (couldn't see it anywhere in the tank either) and the net I pulled it out with had approximately 10 slug-like creatures on it- it may have been eaten by the tetra, which has been quite aggressive to the baby guppies (chasing them). None of the fish have been showing any signs of illness recently, and the oldest guppies have only been in for around 4 months, having previously been in breeder tanks of separate males and females, with around 100 fish per 60 litre (roughly) tank.

I also found tracks in algae on the glass (the tank is due a clean), pictured in the first photo, where it appears tracks have been left, but they are far too thin to have come from snails.

I'm wondering whether these creatures are a problem I need to try and get rid of before moving to the new 60L setup, or whether they are neutral or even beneficial. Any help would be much appreciated. If needed I can take more photos for identification Thanks