If the fish hangs around at the waterfall it is often an indication that there are not enough aeration.
Your water circulation with waterfall sounds fine at 2500l per hour depending on the amount of fish. Your total fish should not exceed 66,6 cm per 1000 liters, in your case 200 cm in total. That would be approximately 10 fish of 20 cm, maximum if your pond is mature. However, your pond is still immature and you should not exceed more than 100 cm in total for the first 6 months. (eg 10 x 10 cm)
Plants will increase the oxygene during daytime, but take it back at night time, so you should not add too many plants.
It is quite common for new fish to go through a stage of not eating. It may take up to three months for them to settle down and get used to their new environment, especially if there are a lot of movement.
Salt helps a lot at this stage and prevents a parasite invasion while they are stressed, creates a mucus layer to protect them from infections and protect them against Nitrite poisoning while nitrites tend to fluctuate as a result of a new pond. Keep the concentration at 3 kg/1000 liters for the first few months.
Their behaviour may also be due to incorrect water parameters. Get yourself a test kit and check your PH, Ammonia and Nitrites regularly and correct/adjsut if necessary. You should have tested the PH before introduction of the fish.
Monitor them closely, and if they flash or hangs all the time, get somebody to take a scraping to check for parasites. They might have Costia due to sudden changes in temperature and stress due to their new unfamiliar environment, and only treat with medication after confirmation of the existence of and type of parasites.