I was rather hoping one of the much vaunted, oft ballyhooed, and rarely heard from Gurus might answer this.
As unworthy as I am, I shall in attempt an answer:
I am working with no information about your tank(s) or its other inhabitant’s flora and fauna wise. I also do not know the basis for your definitive pronouncement on the condition of your tap water and being a potted plant, I don’t get out much, I don’t know what “HEB” is or stands for, I am assuming it is the water purveyor. US $1.50 for 5 gallons of good quality water does not strike me as a terrible price, especially if it is reverse osmosis (RO) or de-ionized (DI) water. Lugging the water around just means, you have no need of some expensive gym membership.
First, I am not entirely sure Glossostigma elatinoides will not grow in your tap water. Have you tried?
I read a comment (I failed to note the source) that growing Glossostigma elatinoides is a good example of the sliding scale of hard and soft water.
I do not have a great deal of experience with Glossostigma elatinoides, what I have found is that above all else they like light, lots of light, same goes for CO2, many start them emerged, me I float them until they grow out, I dislike planting things without decent roots, just irritates me and tears up my substrate. They like good circulation and high nutrients, high nutrient substrate and I have read they like extra iron in the substrate so I provide extra iron.
I have found Glossostigma elatinoides to be one of the least demanding of the ‘demanding plants’ I am not sure why it has such a reputation for being difficult, as with my daughter, give glossos whatever they want and they aren’t the least demanding or difficult.
Just observing my tanks with Glosso, Most are around 6 to 8 dKH (105-140 ppm) and seem to be doing well. I will say that the under 4dKH (70 ppm) tanks do seem rather more lush and dense and harder to contain.
My reading the last day or so seems to indicate a preference for lower KH, but I really have not found anyone willing to be definitive, this is one of the reasons I was hoping one of the aforementioned Gurus and not just some administrator would step in, but it seems to be just me and thee.
To your water, assuming the 13 dKH in your tap water obviously filter or neutralize the chlorine or chloramines and simply cut the tap water with the bottled water. How much to cut really depends on what is in your bottled water.
Another option is a simple filter such as a cartridge filter for an icemaker, get a de-limer, reasonably sure that you are not the only one with these problems in your area. Likely, a local hardware store Tru-Value or ACE could set you up with everything you need to knock the hardness down a bit and rid you of chlorine/chloramines. Just avoid using the water that has passed through a water softener.
Obviously, you could go the RO/DI route, Bulk Reef Supply; 5 Stage Reverse Osmosis Deionization System Bulk Reef Supply
has a pretty dad-gummed good deal going on now. I really don’t think you honestly need that, but it is an option then we can talk about reconstituting and building up the water.