first off my friend 4ft is a real depth you need 3ft of water will chill easy the deeper and more water you have the longer it stays warm. hollow block yes or norm on there side[flat]
get the biggest filter possiable if you plan on using it for your re-build to this is where most people skimp and end up paying the price....
how do you plan on the filter working i see no pump planned for that ???
other than that your uv skimmer etc ok
as for k1 convert there someone here thats done just that on there oasis filter
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