Many water treatment jobs go wrong on the shopping list before anyone opens a box. Equipment that looks simple still has to join your existing pipes, and each join needs a part that matches the pipe size, the pipe material, and the direction of flow. Whether you do the work yourself or hire it out, the fittings list is where the real scope shows up.
Mark every connection point
Walk the path from where the main line enters the house to the faucet you use most. Mark each place the new equipment will cut in: the supply side, the outlet side, any drain connection, and the bypass. At each point, note the pipe material (copper, PEX, CPVC, galvanized steel, or something else) and its diameter. A part that fits on one side but not the other is a common reason a home job stops partway through the day, so measure both sides before you assume anything matches.
Match the fittings to the pipe you have
Connection types are not interchangeable. Push-fit, compression, soldered, threaded, and clamp-style connections each need a particular pipe condition and a particular tool. Check whether the existing pipe is cut square and clean, whether there is enough straight run to fit the connectors, and whether any section needs a transition piece to change material. If you are buying, bring a cut section of the old pipe to the supply counter. If a contractor is quoting, ask which connection types they plan to use at each point.
Confirm the isolation valves are on the list
Every new piece of equipment needs a way to shut it off without shutting off the whole house. Confirm the plan includes a shutoff on the supply side and on the outlet side, along with a bypass or an equivalent way to keep water moving while the equipment is isolated. Check the size of each valve and whether it matches the rest of the connectors. Missing isolation valves are easy to overlook on a shopping list, and adding them after the fact means reopening work you thought was finished.
Write down the small hardware too
Thread sealant, tape, clamps, pipe support straps, and gaskets are inexpensive one at a time and easy to forget. List each one as its own line. Make sure any sealant or tape is rated for drinking water. If an installer is quoting, ask them to confirm in writing which small parts are included, because a line that says only 'plumbing kit' can mean very different things from one company to the next.
Decide who buys the list
Three arrangements are common. You buy everything, the installer supplies everything, or the installer supplies the main equipment while you buy the rest. Each has tradeoffs. Buying your own parts gives you control over cost and lets you shop around, but you carry the risk of a wrong size. An installer who supplies the fittings takes on the job of matching them, and the quote should say so plainly. Whichever arrangement you choose, attach the fittings list to the quote or purchase order so the scope is settled before work starts.
Questions to ask before you commit
Ask the installer which pipe size and material they confirmed on site, and whether that check was done before the quote was written. Ask who covers the cost of a wrong part if one turns up on the day. Ask whether the fittings fall under the equipment warranty or a separate parts warranty, and what each one covers. If you are doing the job yourself, ask the supply counter whether your pipe material and size need a transition piece, and take a sample of the old pipe with you so the counter can check the fit.