Agree that the tank is overstocked & it is quite possible that you are seeing the start of a crash because you are exceeding the carrying capacity of the tank.
For a 20 gal
(80 l), you can keep only the 2 fancies (the oranda & moor). The comet needs to be in a 6 ft long tank (& preferably a pond) since it will get to 12-18" (almost 1/2 a meter) full grown. Plus comets are fast swimmers & need room to swim. Comets in tanks generally don't reach their full size potential and die young. <Normally, comets live for 15+ years in ponds .... the record is over 30 years .... Tank comets don't last more than a few years at most.>
The pleco, if a common, is also a monster. It will get to be bigger than the comet .... You might be OK with a dwarf pleco (rubberlip or bristle nose) in there, but a 20 is a bit small for all 3 fish.
How big are the fish now?? As an interim measure, I would suggest doing water changes to keep all parameters low. The presence of nitrites may be due to something rotting (or accumulation of gunk in the gravel) in the tank. Do a through deep cleaning & gravel vac with the water change & you might see an improvement. Do as much pwc
as needed to keep nitrites under 0.25 & nitrates under 40. You might end up doing pwcs daily or even twice daily. The frequent pwc
's will buy you time & the tank might even stabilize out for a time. But you do need to plan to rehouse the pleco & the comet at some point. <Start digging a pond for Spring?!