Hi freddy when it comes to filtration you want to aim for a filter that can handel double your pond so if your pond is 3000 gallons you want a filter that can handel 6000gallons other wise you will find is an up hill battel keeping the water quality stable i have a 1500gallon pond and i have a filter system that can handel 3000gallons pluse iv got a Bakki shower running as well im going to be going bigger soon to around 4-5000gallons and ill be looking at 1 nexus 300 an easy pod and a Bakki house shower so the more filtration you can have the beter it will be unless you plan on only lightly stocking your pond and to get to your main question for space around the nexus the more the better but id say between 1-2'
