Ok I get you, I keep the exact same species myself. Luckily I have V soft water coming out of my tap but because of the stone I have in my tank my KH naturally rises throughout the week. I think in the case of the fish you have there they're not too sensitive to water hardness, perhaps the ottos but I would say unless you are trying to breed stability and low pollution would be more of a concern than the actual hardness.
What's coming out of your tap, do you know? could you possibly dilute down your existing tapwater with RO to try and reach your target. Maybe you could use pure RO and add something to the tank itself, I read in one of the posts I linked something about maybe adding a little piece of cuttlefish. Maybe a few shells of some kind.
You have quite a small volume of water in the tank so stability will be the hardest part. I have seryu stone (apologies for not knowing how to spell it
) Could you incorporate a couple of small stones somewhere in your scape. I find having these things in softwater release a small amount in the tank gradually which is a bit better than dumping a load of salt in there in one go.
You also need to bear in mind that most fish are captive bred and although replicating their natural environment is a good thing in the case of hardness they are quite flexible at adapting as long as it's gradula. It's more a long term effect issue and what keeping them in water they haven't evolved to live in may have over the long term. I have read of Discus being kept quite happily in liquid concrete
only softening the water down for breeding purposes. The TDS is probably a better gauge of hardness than your KH/GH measurements anyway way. In my case in my main tank dosing EI my GH 4 and KH 3 which you would assume to be of very low hardness but the TDS measures the day before a water change at 250ppm which is moderately hard water. That's what the fish will feel if you like. Luckily I have tapwater coming out at 40ppm so it's just a case of doing some extra water changes to reduce this down when I feel it's getting too high.
Not sure whether the build up of TDS is through the salts I dose, pollution levels or both but either way it's best kept down for softwater fish. From what I understand living in extreme hardness conditions which they are not used to has a detrimental effect on kidneys of the fish which regulate the salt content of the fish to keep it in balance with the surrounding water. Small deviations between 1 and 2 degrees as long as gradual shouldn't be an issue unless they were dumped straight from one to the other which is why fish need acclimatised when going in new tank.
PH is a different thing, from what I understand PH affects the blood of the fish transporting oxygen around the system so big swings can do serious damage. Adding KH in a lump can cause a quick PH swing hence it's probably better practice to either have something in the tank that does this gradually or to add a small amount of your Sera stuff daily which might help in it dissolving. Someone mentioned earlier that you need acid water to dissolve it, that may be the case with the acid reacting with the carbonate to break it down so dumping it all in maybe won't help.