Right, ok now we have an idea of what’s happening with your tank and plants then.
Simply put, the plants needs more than what you’re giving it at the moment.
If you’re new to the fishkeeping hobby or new to live plants in aquaria then I would not suggest setting up a high tech with Co2 system set up etc,
Perhaps I think best option would be to get some new easy to care for type of plants and discard those in the tank as they’re pretty well gone if nothing extra can be done for them, sorry
You’re certainly not alone in having had this happen, it has happened to me before as well and just have to learn from it and chalk it up to experience.
It looks like there is no livestock in your tank from what I can see form your pictures, is it currently going through a fishless cycle?
Cycling Your First Fresh Water Tank What is Cycling and Why is it Important? Fish waste, and especially fish breathing, plus uneaten food and other organic matter breaking down in a tank all produce Ammonia. This can quickly become toxic to fish if it is allowed to build up to any measurable...
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And suggestions for easy or low demanding plants will be along the lines of vallisernia, cyrptocorynes, java ferns, anubias, sessiliflora and there are lots more varieties of course, a case of seeing what you like and doing a little research or post a list of plants you like and we could possibly advise on those.
Also I would advise lowering your daily lighting times to around 8 hours, this tends to be the sort of time where it would not impact on plants too much and not to encourage algae growths, of course this depends on a few variables but generally that’s what tend to work.
Some direct sunlight can be bad for aquarium as it can increase the chances of algae but not always, generally is not advised to have a tank in direct sunlight but it depends really.
One other thing, increasing your weekly water changes from 25% to at least 50% once a week, probably better at around 70% really, water quality is very important and doing late get water changes replenishes minerals and of course clean water for everything the tank livestock and plants requires, of course dechlorinator / tap safe to make it safe for the livestock.
Sorry if these are not what you wanted to hear, no quick fix solutions for your current plants am afraid, and quite a bit of advice, it’s bit of a learning curve really.
