I agree with the others. I've seen bloodworm compared to a Macdonald's burger - fine for a treat but you wouldn't want to live on them. Unfortunately fish regard bloodworm the same way children regard a burger - given the choice they'd rather eat them than nutritious food. You may have problems weaning them back on to flake. If they won't eat them, persevere. Remove the uneaten food, then feed flake again next day. Don't give them any more bloodworm until they will eat flake. Fish will not willingly starve themselves to death, they will eat the flake eventually. Then you can give them bloodworm once a week as a treat.
Fish don't have flakes in the wild but they do have access to a wide range of live food, wider than we can supply to them.
Platies especially do like veg in their diet. You could try getting a veg based flake food to mix with the standard flake, or feed peas once a week (cook, remove the skin and chop). Or a slice of courgette, blanched for a minute in some water in the microwave to make it sink.
Re the filter, the media only needs changing when the sponges fall apart or won't go back into shape after squeezing. If you have cirax, that needs changing when it starts to crumble. The filter wool does need changing regularly with the Juwel filter as the wool is first in the direction of water flow and gets blocked with debris pretty quickly. Because it is changed so regularly, and it doesn't make a very good home for bacteria, you remove only a tiny portion of your bacterial colonies when you change it. If you are using Juwel white pads, get a roll filter wool and cut that up using a Juwel pad as a template. Much cheaper.
The reason the instructions and shop say to change the media is to make money. But the shop was right in that when you do need to change the media, you should only change one bit at a time with at least a month between.
