good luck catching the loaches out of your tank if you want to return them, 800 litres is a big tank
I would add a few more loaches so there is a group of them.
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Fish don't eat to keep their bodies warm because they take their body temperature from the surrounding environment (water). This means any food they eat is used for moving about and growing. Because of this fish can go for weeks or even months with no food. A couple of days without food will not harm them at all.
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You could try the snail trap from Nick's post and put the food in it. The catfish will not be able to get the wafer but the snails will.
I have never used No Planaria but it is made from Betel nut, which is a plant from New Guinea and the people chew it for a buzz, a bit like tobacco. So it should be safe.
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You mention doing water changes every other day. Why are you doing so many water changes?
If it's to control snails, water changes don't do anything. If you have an ammonia problem then reduce feeding to twice a week and do 75% water changes and complete gravel cleans.
In a big tank like yours, I would use a 2 litre plastic drink bottle and a length of plastic hose as a gravel cleaner. It will be quicker and more effective and you can run the hose out the door onto the lawn or garden so the water is not completely wasted.