[Zoidburg]

Can't respond in full... but in short... you *DO NOT* have hard water! 6 GH is, at best, "medium" water, but I would personally consider it soft. My sister has water coming out at 19 GH and 10 KH...

High pH does not equal hard water. Likewise, low pH doesn't mean soft water. There's a lot more that plays into pH and water hardness than what's often believed.

Switching to RO/DI would be fine... however, you need something to remineralize that water to the recommended parameters. Straight RO/DI (or distilled) would be a death sentence.


What are you using to test your water parameters? Is it still 'in date'? (not expired?)

 

[Zoidburg]

Thank you for all of that info! That clears a lot up. Does a high TDS in straight tap water mean that the water is “hard?”

Not necessarily, no.... but generally, yes.

TDS can also be made up of food, poop, dust and many other things... which means you could have high TDS while having soft water. The harder the water is, the higher the TDS will be, of course. 1 GH/KH equals no less than 17.86 TDS, but that value could be higher... i.e. 1 GH could equal 25 TDS. It depends on what makes up those minerals and how they dissolve in the water.

And should I expect my heavily planted tank to have a higher TDS in general than one without plants (assuming the same stock levels)?

Not enough info to say.

Rocks that release minerals can raise TDS.

Plants can help keep the water column clean of ammonia, nitrites and nitrates.

Buffering substrates can reduce KH and/or GH and thus TDS.

Unless you measure out exactly how much food the fish eat and have it same per tank, and have the fish actually consume all that food the same in each tank....

Well, lets just say there's a lot of variables.