So, i have several thoughts on this.
A concentration of ~40000 molecules per Liter of water is absurdly low. To the point where there is absolutly no scenario in which it would be viable to filter out. 1 Liter of water has this many molecules in it, 3300000000000000000000000. For Comparison.
Then there is a conceptual problem. We know all elements up to 118. And we know Elements beyond Lead tend to not be stable. And generally, the higher the Atomic number the more radioactive elements become. Even if you argue Teaterium is "only" Element 119, optimistically the half-life is measure in nanoseconds. So the notion of Teaterium not being screamingly radioactive does not hold.
Then it is also questionable why this would be good for Fusion. Fusion is all about overcoming the repulsion forces which make sure Atomic nuclei do not just merge. The bigger the nucleus is, the harder it is to fuse and at some point Fusion takes more energy than it releases. That cutoff is at Iron. There are certain situations during which the fusion of any element is possible, but like we are talking about Neutron Star collisions type forces at that point.
To address these concerns, i would advice to like raise the concentration of this stuff a lot, and make it an Isotope. So some variation of an Element with extra Neutrons. A lot of Isotopes are sort of stable, for instance my favored element Lithium has like at least 13. Some, like Lithium 6 (Because Irony) and Lithium 7 are stable. Your whole setup would make more sense if it was a Isotope of something common. Like Lithium, idk Helium or Magnesium.
Even then, Fusion has another issue that makes it pretty hard to get the "Perfect" Fuel. The very nature of Fusion is to ram particles at high speeds, under some pressure and absurd temperatures together until they stick. That process will never be without Radiation. Even if you Isotope / Element is Aneutronic (So the fusion itself does not generate free Neutrons), other effects like Bremsstrahlung will generate Radiation regardless. Bremsstrahlung btw is the radiation particles emit when their momentum is changed. Essentially, the harder of a U Turn a particle makes, the more radiation it emits.
We know, the moment you leave the Hydrogen domain of Fusion fuels, the reactivity goes down. So you need higher pressures and or temperatures to regain efficiency.
This graphs shows quiet well why for Instance He3 + He3 fusion is not going to happen. Helium 3 Fusion, even with Deuterium, is so endlessly less reactive than Deuterium + Tritium that it is not a contest. The types of reactors that can get a net energy out from DT Fusion are lightyears from achieving any net gain with D-He3. If your Element is anything other than another Hydrogen Isotope, it physically wont we able to generate any better Fusion than what we can already do. Which is not great.
So the best properties you can give it be about as good as Deuterium or Tritium. You will note, Deuterium and Tritium are literally the second and third isotopes of Hydrogen. As far as i know, the only empty spot for you would be Hydrogen 8 and upwards. But the trendline is that those isotopes have a short half life.
So what to do ? After all, so far my answer has been pretty down beat. Honestly, i think you should just not worry to much about it. Say its an Isotope, say it is found in nature, say it has good Fusion properties. Like that it has a high reactivity at "low" temperatures, the products it creates are not Radioactive themselves and its Aneutronic. That will pass for most people.