What causes illness if water conditions are fine?

[VikingMummy2015]

How do you acclimatise new fish to the tank?

usually just let the bag of new fish sit in the water for about 30 mins/an hour. then every 15/30 mins add some of the tank water to the bag until the bag is then full over a few hours and then sinks/ or once its near ful i tip it horizontal to encourage the fish out while under water

For future fish, don’t contaminate your tank water with bag water from the store as you could introduce anything from that water. Instead, pour the bag water out into a bucket over a net to catch and then transfer the fish into the tank.

[plankton]

@VikingMummy2015 has put what I would have put.

Also it may be worth testing the bag water to see how different the ph and hardness is to the tank water. (I do this regardless, using a TDS meter -cheapish from Amazon if I remember - which is basically doing that ) You'll then be able to see when it's actually balanced.