Hi!
There are some corydoras species that can live in harder water - off the top of my head since I have hard water and had help researching, bronze corydoras and sterbai cories can tolerate quite hard water, I keep both, and a group of six of either of those species (one or the other, or six of each, but that would be overstocking in a 20L. But a group of six of either of those would work, and there are a few more that can manage harder water too, but I can't remember those right now I'm afraid. Most, like pygmies, do need soft water, and are likely to have shorter lifespans and less likely to breed if the water is too hard for them, since the excess minerals they retain cause internal blockages, like blocking the kidneys.
So yes, the soft or hard water issue is important, as you've already discovered with the shrimp moulting issue, and you researched and worked to correct it, which is awesome!
Having said that, I don't think your water is that hard, in the fishkeeping sense, and might work for pygmies! I'm not sure, need
@Essjay or someone else who is smarter than I am like
@Seisage or
@GaryE to do the conversion for the GH for us, if you guys don't mind!
Seriously Fish is a reliable and accurate website that has profiles for most every fish you'll come across in the hobby, and is really useful for checking things like the GH range, temp range, pH range, and their general habitat, preferences, tank sizes, and compatibility.
For pygmy cories, Seriously Fish gives this range:
Temperature: 72 to 79°F (22 to 26°C)
pH: 6.4 to 7.4
Hardness: 2 to 15°H but preferably below 8.
Here's the link to the SF profile if you want to dive deeper! I don't know how to convert the H to GH, but I
think that your 150 GH might be within that range. Not totally sure on that though, so if you can work out the conversion, or wait for one of the people I tagged to figure it out, then it might be fine.
Pygmy corydoras are wonderful little fish, so it's a good choice, and a 20g long with that larger footprint is great, you can do a lot with a tank that size and shape. I wish they were more available in the UK!
My tapwater is pretty hard, at 253 GH, so my larger tanks have hard water tolerant fish like mollies and the above mentioned bronzes and sterbai, but I have a smaller tank with a breeding colony of pygmy cories, and some otocinclus which I keep softer. I can't afford/deal with the hassle of RO, but I fortunately have water barrels that collects rainwater, and I do a 50/50 mix of my tapwater and the rainwater so the GH is lower in that tank for those soft water fish, and add botanicals like oak leaves and alder cones to add tannins and lower it further. It's my compromise, since yes, the vast majority of fish in the hobby evolved to live in soft water.
But you're fortunate to have softer tapwater, gives you a much wider range of options! If the current tanks harder water turns out to be too hard for pygmies, it's the perfect excuse to set up another tank. That's how we wind up with MTS - Multiple Tank Syndrome - a serious and common syndrome in this hobby!
But platies certainly need hard water, and tend to fail to thrive if the water is too soft for them. Worth looking up all the species you have, and any you want to add, on Seriously Fish. Make a note of their hardness and temp range etc, and compare, to make sure their ranges all work with the hardness of your water. Gives you a much better chance at long term success. But it sounds as though you're doing well and making thoughtful choices, and wanting the best for the fish, and that's the most important thing.
Would love to see the tank if you feel happy to share a photo, and welcome to the forum!
