I suggest that you forget everything you were told, everything, and start again. Here's a good place to start.
http://www.skepticalaquarist.com/parasites-protists
I don't like fine sand like pool filter sand for many reasons. If you're going to go with sand, consider the sand that Cory sells. It's not so fine that it blows around, gets caught in your filters quite so easily, and forms nasty gas pockets etc.
Columnaris, Flavobacterium columnaris, is a bacterial infection, not a parasite. Different bacteria thrive in different temps but higher temps are generally beneficial for bacteria.
"Columnaris thrives in temperatures above 80 F (I have observed 85-90 to be a range where Columnaris is most...
It would be simple to section off the sump with some foam = make a matten filter, and then use any pump you want. Fish would be protected.
http://www.seriouslyfish.com/a-basic-gu ... tenfilter/
Can you draw water where it enters the building, before it circulates through the system? If it's an older building especially the copper may be leaching from the pipes as it circulates through the house.
I wouldn't chance using them if they're that far off. Time for new heaters. I've been having issues lately also with the Eheim heaters which I thought were going to be good. One went after 1 year and 2 more after 2 years.
4 weeks minimum. I like 5 & ten gallon bare tanks as they take less meds and are easy to manage. But I don't keep large fish. All new fish get at least 5 days flubendazole treatment followed by 5 days quinine.
That's a wild looking filter for the price! But for your purpose I think you'd be better off with a dedicated UV filter. That 9 watt bulb at 525 gph won't do much. The flow rate must be balanced with the bulb intensity to kill anything. Have a look at this chart from Foster & Smith...
What are you measuring? Soft doesn't equal acidic. Equalibrium will supplement your gh(calcium/manesium) and baking soda your kh(carbonate). Kh is your PH buffer. My tapwater is 1-3 for both kh/gh and tds about 50. Ph 7+ By adding what I do I keep most tanks around 100 tds and the ph remains...