Fish hormones with no fish? I hardly feed the tank so how do trace nutrients build up?
Dw I still do 30% weekly water changes, I'm just wondering the specifics on why people do them
The only information initially given was that you'd added shrimp to a 40g tank, not that there were no other inhabitants
And trace nutrients could still build up through decomposition of plants and through ferts if they're used. Additionally, many people will encounter problems of mineral build-up if they're not changing water but are topping up the tanks with tap water.
If TDS is the way for you, then go ahead with that. Much like @Asteroid
though, I'd rather just do the water changes proactively - I'd be trimming plants and siphoning sand etc. anyway, so why not just do the water change? But that's just my personal situation and outlook.
Some more anecdotal advice here - I buffer my tap water from kH 0-1, gH 2 to kH 2, gH 5 and perform regular changes as previously discussed. However, my main tank went without a water change for just over 10 days and the pH dropped from 7.2 to 6.8 as the kH was depleted by the cycle. That's not a huge amount (although pH is logarithmic) but it could have led to a full pH and cycle crash if left for more than a few weeks. This is all dependent on the parameters of your water, but as @DaveKS
said, keep an eye on other parameters too, even if you are testing TDS.
Not saying that any specific method is the "best" as it varies so much on circumstance, but these are the specifics on why I do them