sorry, i guess i worded it poorly, my tap water is pretty soft, however im worried about a substrate driving up hardness, im sure Tom had this problem in one of his journals?
I do not recall that Journal, but that might be i didn't read them all, Tom is an old time UKAPS member from long before i joined the club.
But if any substrate drives up the hardness it could be premixed with Calcium and or Magnesium, some dirt soils from the garden centres are premixed with that stuff, than the contents on the lable should say so.
Regarding dirt soil i have very good experience with Velda Lelite, it specialy developed for Lilies in the pond.
https://www.velda.com/pond-products/bottom-plants/plant-substrates/lelite/
It contains blue loam and dried fenlland peat, this makes it a very steady compact but still rather soft soil and it stays long time fertile.. I have one aquarium setup with this alreaddy for the 3th season now with a bunch of rather fast growing plants and still didn't need to add any additional fertilizers till now. Plants grow mental on it..
But still regarding creating a soft acidic condition, it might do for a little while depending on the tap parameters because of the peat it contains. But peat will buffer and somewhat soften your water and lowering pH. But it isn't permanently, at one point it is done buffering and it stops altering water parameters. Than for example most tap water is /- nutral in pH around 7, since it is under pressure it contain some extra CO² affecting the pH. After 24 hours not beeing under pressure anymore the water will be depleted from that extra CO² and rise in pH above 7. If teh peat in the tank is done buffering and saturated than you have to add new peat again. Mean to say in the substrate it can't be done, than you need to put peat in the filter etc. In general, theoreticaly it is said, peat for altering water parameters needs to be repalced +/- every 6 to 8 weeks. If you don't than regarding pH you will be stuck with what you get from the tap. If peat alters you water parameters is highly depended on what comes from the tap..
For me personaly, since the usual recomeded dosage of peat doesn't alter zip for me other than making the water coffee brown, this also was a point of consern with my idea that i can never keep any Apistos. But regarding other experienced UKAPS members it aint an issue because all captive bred Apistos in the trade are addapted to live healthy in higher pH vallues than their wild caught cousins are.