If frozen foods are too big for the fish, use a pair of scissors to cut the food into smaller pieces.
If the picture of the tank with harlequins is the entire aquarium, I'm not sure there's room for more fish. You also have a blue green algae (Cyanobacter bacteria) problem, the dark green slime on the plants, wood, rock and substrate. This is normally caused by excess nutrients (uneaten food, particularly dry food) and unused plant fertiliser will contribute to its growth. Cyanobacteria also like red light, low oxygen levels and slow water movement.
To treat blue green algae (bga), do big daily water changes and gravel clean the substrate every day for a few weeks. Try to remove as much of the bga as possible. Don't use plant fertiliser during this time. Reduce the amount of food going into the tank. Increase aeration and water movement, especially around the bottom half of the tank. If you use fluorescent light globes above the tank, try to have globes with a 6500K rating. LED lights are normally fine but have a 6500K rating (cool white globes) on them too.
If you still can't get the bga under control, get some "Ultralife Blue Green Slime Stain Remover".