Agreed that it's most likely a water quality issue that has them surfing in the corner and losing a few. The skeleton being stuck to the filter is just likely where that skeleton ended up since it's a filter, not an indicator that it was a cause of death, IMO.
Since the tank is well planted, the fish in question are a small size, and there are other fish, shrimp and snails there to consume small bodies rapidly, it's very possible to have a few losses without it leading to an ammonia spike, fortunately. Always best to do a search for a corpse if a death is suspected of course, and to remove a body if you find one, check water quality etc, but just letting you know that especially in a tank with thriving plants, small fish and other hungry inhabitants, that it's easy for small fish to be rapidly consumed by the others without any water problems as a result of the deaths. The soft skeletons are also usually broken down and consumed pretty quickly, so you're not always going to find the evidence from a vanished fish.
Of course, that applies to the odd loss, and the cause of death. If the death was a result of water quality issues, then it can quickly escalate as more fish begin to pass away, and the more corpses that happen in a short space of time can quickly lead to a tipping point where more ammonia is produced than the system can process and lead to a full tank crash. So you're doing the right thing in investigating missing fish and potential causes of death.
It is a beautiful tank and the plants look as though they're establishing themselves really well! But I'd urge water changes and not just topping the tank up. There are a lot of things that can go wrong when your tank water chemistry drifts too far from the source water chemistry, and old tank syndrome as a result of only topping up tanks can lead to some nasty emergency situations where your hands are tied to fix them without massive losses.
Since the tank is well planted, the fish in question are a small size, and there are other fish, shrimp and snails there to consume small bodies rapidly, it's very possible to have a few losses without it leading to an ammonia spike, fortunately. Always best to do a search for a corpse if a death is suspected of course, and to remove a body if you find one, check water quality etc, but just letting you know that especially in a tank with thriving plants, small fish and other hungry inhabitants, that it's easy for small fish to be rapidly consumed by the others without any water problems as a result of the deaths. The soft skeletons are also usually broken down and consumed pretty quickly, so you're not always going to find the evidence from a vanished fish.
Of course, that applies to the odd loss, and the cause of death. If the death was a result of water quality issues, then it can quickly escalate as more fish begin to pass away, and the more corpses that happen in a short space of time can quickly lead to a tipping point where more ammonia is produced than the system can process and lead to a full tank crash. So you're doing the right thing in investigating missing fish and potential causes of death.
It is a beautiful tank and the plants look as though they're establishing themselves really well! But I'd urge water changes and not just topping the tank up. There are a lot of things that can go wrong when your tank water chemistry drifts too far from the source water chemistry, and old tank syndrome as a result of only topping up tanks can lead to some nasty emergency situations where your hands are tied to fix them without massive losses.
