There are a couple of things to consider here. The first is whether you actually have hardwood floors, or laminate flooring. I mention this because almost all laminate "hardwood" flooring has actual wood veneer on the surface, so it can be difficult to ascertain whether it's actual solid wood or an engineered product just be looking at it.
A little water will not damage an actual hardwood floor, though it may disturb the finish if it's a film-type finish such as polyurethane. Likewise, a little water won't damage an engineered product either. The difference is in how fast the floor dries. A real lumber floor won't be damaged from water actually soaking into the wood provided that it's allowed to dry out over a few day's time. Water soaking into an engineered floor will severely damage it from expansion of the OSB/MDF backing under the show surface.
The bottom line is that you can certainly put a polyethylene tray under a reef tank, but you must be exceptionally careful not to get water under it that would be trapped, especially if it's a laminate floor. Another solution, though, is to carefully shim a modern panel-type tank stand all the way around the perimeter so that there's a 1/8" gap between the bottom of the stand and the floor. That will allow any spilled water to evaporate.
