Mildew and Mites

[Alex]

So I have had some issues with mildew and spider mites in my new system.  It's a small system on a 10 gal fish tank.  

Just a few questions that I haven't had luck finding the answer to through the search feature:

To deal with the spider mites, I plan to just replace the plants as they are only some herbs that I got at the grocery store.  I will also be looking into worm casting tea to rid myself of mites long term.  Question is - if I get rid of my plants and immediately replace them will I soon have mite problems again since I haven't eradicated the problem first?

Also - I've had some success with some of the mildew remedies I've found on here but it hasn't fully gone away because my hydroton is too shallow.  My question is - can I spray the mildew and immediately add more hydroton or should I ensure all of the mildew is gone for a certain period of time before adding more on top?