I have had my Chi for about a year now and can maybe shed some light on this.
I have about 4 shrimp in the tank now and did have more, but with the open top I did find my shrimp would leave and I found one who died underneath the couch. Since then I've added a lid and have had no problems. Consider it a warning as I've also had fish leave through the open top and have this is not uncommon. Yes- I do change water regularly and get it checked to ensure the quality is good.
I am looking into replacing the filter to provide a better environment for the fish and like your setup there.