Welcome to FishLore

I'm really sorry to hear you lost your Angel.
It would help to know the readings like aquarist asked
According to your aquarium info, you've had this tank set up for a year?
Have these same fish been in that tank for that long?
28 Liters isn't big enough for the 3 fish you have.
This may be why they seem to be acting sad, they don't have room to grow and act as they normally would in the correct tank size.
Do some research on the fish you have remaining, it would be a good idea to look into getting a larger tank.
Clown loaches like company of their own kind, but they get really big. I think for a group of the 75 gallons is recommended.
Good luck.