I use
Seachem. Purigen on my FW planted tanks, it says for use in marine and FW. Is absorbs dissolved organics, but I think it's more for circulation in the tank than purifying the water during PWCs.
Our tap water here (des moines ia) is over 9.0 out of the tap but drops fast in the tank, we have a drinking water RO/DI but it still comes out over 8, high phosphates and silicates as well, so I've got a similar problem, I get a lot of algae in my FW planted. Since I added more plants it has dropped considerably however.
As for SW, I don't know of anything short of buying distilled water or RO/DI. You can buy it in parts and DIY or shop online, but i don't know the cost. Lucky for me the tank I bought from someone getting rid of it came with an RO/DI but I'm sure I'll have to replace the media just to be safe.