I've got a Nexus 200 and have K1, K3, biochip and now have just introduced micro K1 in the bio section.
My thinking is I'm offering a variety of differing surface areas for the bugs but that may well be pie in the sky thinking.
Any way all the media works well with in my opinion bio chip being the best for bug colonisation, the only problem with bio chip is that there's a very slight chance it can escape through the grills and end up in the pump but in 4/5 years I haven't experienced this.
I've introduced micro because I need to up the biological section to cope with the fish growth and micro offers a large surface area in relation to the space it takes up.
Despite a few comments I've read micro is too large to escape through the grills of the older Nexus.
If I could I'd have a lot more bio chip but I don't think it's available anymore because of the problem I've mentioned.