Home filtration systems generally DO NOT go to 0 TDS because of the taste of the water. Pure water isn't quite as yummy tasting as it sounds. Even bottled water is about 300 TDS (Figi water says on the bottle what theirs is). No matter how good your home filtration system is, it's not suitable for aquarium use (more than likely.)
Please let us know what he finds. In all sincere politeness, please do not obtain animals that you cannot keep thriving. If you've had a problem with clams in the past, claim to not know anything about clams, and do not have a large enough system for a Gigas clam, then please wait until you are ready.
Purchasing animals that require specialized care and not providing that care can ultimately lead to trying to find a new home for it...it would probably go to someone that also did not know how to care for it...and it will eventually die. When masses of people follow this pattern, more harvesting of these animals will occur...and then the local pet store will be saying that anyone could keep them in a 29 gallon nano.
Ok, I'm off my soapbox. This wasn't directed entirely toward you, but for anyone considering large or specialized animals with the intent of "upgrading". Everyone hopes for it, but rarely does it happen...instead, life happens.
Speaking of having someone come look at your system...every month we need someone to host a club meeting. If you were to host, you'd have lots of knowledgable people able to get a first hand look at your system. It's a lot easier for unfamiliar eyes to spot the problem!