There's an article here on using RO: https://www.fishkeeping.co.uk/articles_109/ro-freshwater.htm

If the only problem with your tap water is hardness, you can simply mix a proportion of tap water with RO to get the parameters you need. I remineralise instead because my tap water contains a high level of nitrate which I don't want to put in the tanks. A very easy to use product is TropicMarin Remineral Tropical. You just need to add enough to get the TDS where you need it to be - around 150 for most tetras. It's readily available on eBay.

Thank you, again very helpful.

Does anyone gave any recommendations for the best water testing kit for a tropical tank?

The API Freshwater Master Test Kit sounds like a good one - but us it adequate? The last comment in the review I read said: “this kit is usually enough to monitor and maintain a simple tank” which actually makes me wonder if I need something more comprehensive!

The API Master Test kit is what most of us use. The only things it omits are gH and kH which can be bought separately if required. If you do go down the RO route, you'll also need a TDS meter to check your RO so you can tell when it's time to replace the membrane and to measure how much minerals you're adding.

Thank you 😊

been watching youtube for how to install my ro system and a DI (de-ioniser?) cartridge was mentioned. Is this necessary for a tropical fish tank or not.

Thanks

No, you don't need a DI stage for freshwater fish, it's used for marine tanks.

Thank you...again