Did I do something wrong?
Sorry to hear of this, your photo shows 0 total hardness. That alone, if accurate is a recipe for disaster. Did you partially remineralise your RO water?
First kept fish seriously age 8, still keeping fish in my 60s. Here are my observations which you may find useful.
I had books from the library to read, my parents insisted, before they bought me a tank, buy a good book and treat it like an instruction manual. I strongly recommend Karen Randall but there are more basic guides out there, Karen is a plant enthusiast. Plants are a challenge, they need CO2, fish need O2! Do not believe the twaddle that a well planted tank is always safe for fish, it is not true of lakes or rivers, it is not true of aquariums. I have seen fish die in the summer in heavily planted lakes.
Sorry, no offence, but topical fish are relatively easy, at junior school I had several friends with tanks, we all managed to keep fish, sometimes we got whitespot, ich in the US. But, plants, that was a different matter. But the good news, floating plants aren't difficult, but seriously some rooted plants can be very difficult to grow.
Fish must be disease free and suitable for the water/temperature and size of tank. Angel fish for example really need a very large tank, 5 or 6 feet. Yes 5 or 6 feet, don't trust shopkeepers who say otherwise. They are large and aggressive cichlids, they fight, they need territory and their BO demands are significant - biological oxygen demands.
Best to use tap water.
Seriously, just use tap water, only be aware of chlorine products - in the UK we generally just let water stand for a day or two, we don't normally have chloramines in our water but there are exceptions, Scotland I have learnt today.
Keep it simple and keep fish and easy plants first - treat for the chlorine/chloramines but otherwise use tap water.
I stress, use tap water, and stop testing your water.
Use floating plants, use a filter and heater and moderate light for 8 hours a day.
Do not try and adjust the KH, pH or GH, just go with what you have, lots of fish and lots of plants are happy in tap water so long as it is the right temperature and types of chlorine aren't present.
If you can, use filter material from a mature tank, if not, run a tank with no fish for weeks. Mature means running well for months not weeks.
Introduce small hardy fish that actually like your tap water parameters - hard water in your case, guppies, barbs, rasbora species. Stop thinking water has to free of hardness.
Use hardy plants that like your tap water, hornwort, Vallis, egeria, hygrophila.
There are also some crypts that like hard water. Amazon swords like hard water. Do not add any plant fertiliser at all until the tank has been running with healthy fish for at least 3 months and then only go light, and don't over clean filters, dirty filters are your friend, they are full of bacteria. They work best when slowed down by brown gunk.
Do not even think about CO2, high light or messing about with water hardness until you have run a tropical planted fish tank with no losses from disease or water issues, for a year or two. And then, go gently, CO2 and water softening is not for folks new to the hobby unless they have a scientific background.
Sorry again and best wishes, but luck is not needed just patience and small steps, we walk before we run.
