I don't know if this is really an answer, but I've had a Coleman 6 person spa in service for about 25 years. Yes, 25 years! I have never had the same problems you are experiencing. We learned early on that any clothing (bathing suits etc) must be very clean and rinsed extremely well before going into the spa. Clothes cleaning detergents left in the fabrics release into the pool water and cause foam quickly. The clarifier is good at flocking these soaps, but they do tend to gum up the works quickly. We never lost a lot of pressure however, just had to clean the filter. We also made a habit of completely changing the water at least twice a year. After draining as much as we could via the drain hose, we used a wet vac to get as much out of any low spots. We then put in just enough water to cover the intake close to the bottom and jogged the pump to cycle clean water through the system and get the dirty water out. Again, drain and vacuum and repeat this process until the water looks clear. We do this with the filter out. This will clean out a lot of crap. About once a year just before we do a drain and complete cleaning, we treat the tub with anti-scale cleanser. This cleans the internal pipes of built up scale and solids. In your case, you may want to do this anti-scale treatment for a few days with the circulator on low and the filter out to purge the internal system. Then go ahead and change the water.
Another test you can do is to see how well the pump pushes water. When you have the small amount in the bottom, just above the intake, jets exposed, turn the jets to full, air off and see if the pump will shoot the water out of the jets with extreme force. Mine will shot water 20 feet away!!!! lol. If you don't have really good pressure, you may have a warn pump impeller. They can be damaged over time by chlorine based sanitizer, I never use chlorine, just bromine based shock etc.
The other item to check is if you have the right filter for your unit. Filters come in so many styles and densities. You may need to find one that allows better flow.
Assuming you find the root cause of your problem, good housekeeping, keeping as much contaminates as possible out of your tub will help. Very clean bathing suits or none at all,(yahoo) no body cosmetics such as body oils, moisturizers, perfumes etc. Keep chemical treatments to a minimum. Do a quick check weekly for PH, Total alkalinity, and bromine level. Adjust them sparingly so not to overdo any chemicals. Once you get everything under control, maintaining it is very easy and quick. Good Luck
Though it may seem odd, it is possible the plumber knows what he's talking about. If this is a Polybutylene pipe, it could indeed be brittle and fail if it's jostled during the installation of the filter.
It's impossible to tell from the picture what type of pipe that is, but based on the plumbers concern I'd guess it is Polybutylene.
Polybutylene
Polybutylene was used in pipes between about 1980-1994/5, and was commonly used as supply line in the 80's. It was found (in some cases) to react with oxidants in the water supply, which caused the pipe to become brittle and fail. To my knowledge, this type of piping is no longer used. Though if the house was built; or there was supply line work done, during the 80's, it's likely this is Polybutylene pipe.
High Density Polyethylene
Don't confuse Polybutylene pipe with Polyethylene pipe. High-density polyethylene is commonly used as water supply line today, and is stable and safe to use.
Cross-linked polyethylene
Cross-linked polyethylene (PEX) is not typically used as an underground supply line, as it can sometimes react with minerals in the soil. Where PEX is used underground, it's typically encased in sand (or other non-reactive materials) to prevent negative reactions. Because of this it's often more expensive to bury PEX, so other materials are used instead.
How can I tell what it is?
The only way to be sure what type of pipe this is, is to look for markings on the pipe itself. Polybutylene will be marked PB####, Polyethylene will be marked PE####, Cross-linked polyethylene will be marked PEX-x (where x = A, B, or C).
Best Answer
So if I understood you correctly then you want to dig in the ground, raise the water pipe to a filter, then back down into the ground?
I have to say that doesn't seem like a good idea to me. If I were doing this I would just cut a pipe in the basement and put the filter there. In particular I would cut just the pipe going to the kitchen, and not the whole house.
But if you really want too...
For the enclosure use double layer (4 inches) extruded foam sheets including on the ground! Then cover them in some exterior-grade plywood on the outside to protect from damage.
i.e. build a plywood box without beams/poles inside it (put them on the outside), then glue the foam sheets to the inside. Lay some plastic on the ground with small holes for the pipe, tape the plastic to the pipe, then foam sheets on top of that, and put the whole plywood box on top of it.
You'll probably need a 3rd hole for some sort of support to hold the filters.
I would suggest you wrap the pipe first in electrical heating tape (they sell it to protect pipes from freezing, it's what you want) then in waterproof (i.e. closed cell) foam rated for underground use several inches think and bury it.
Taxes is rather large - if you happen to live in a place that doesn't get very cold then this might not be necessary. I checked and record low in taxes was -23F, but that's probably not where you live.