Ok, have had a look at your other threads, (one about cardinal tetra and one about a platy) it looks like you’ve unfortunately have had some issues so far
And sorry to hear of your loss of your betta as well
From what I read from your other threads and this one, back in May you did not cycle the tank when you first got the tank, LFS told you to add fish straightaway, this is very poor advice from the LFS btw.
So therefore at this point I am unsure if the tank is cycled or not even though you got the tank a few months ago.
Have a little read of this to help give you an idea of what we mean by cycled, it’s a bit of reading and quite a bit to try and understand first time round but read it a few times, that should help.
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...
www.fishforums.net
So what we do actually know so far-
Your tank is 72 litre.
(which is just under 20 US gals)
But unsure of dimensions/measurements as this can be important to know as certain species of fish requirements differs, some species could either need larger, longer, taller or even smaller tanks, it depends.
You have some platies, mollies and cardinal tetras and possibly a Red Tailed shark but this is not confirmed.
We kind of need to know what livestock you have exactly in your tank currently as it can affect your water quality in your tank for example.
And you said on one post on the other thread that your general hardness was at 120m/gl , this is moderately hard water.
And you have plastic/fake plants, no live plants.
And from your other thread I read that you have a dip strip testing kit, not the best test kit am afraid.
We can recommend a half decent testing kit which API Master Freshwater test kit, a bit more pricey than the stop dip test kit but this recommended kit can help give much more accurate reliable water parameters readings of ammonia, nitrite and nitrate, we will then have a much better picture of your tank parameters and whether it’s cycled or not.
So we have some clues but not the whole picture to be honest.
Very hard to know what advice to give without seeing your tank and testing for ourselves so therefore we have to rely on your information that you give us, the more information you can provide, the better.
So do carry on doing the water changes of 70%, and do follow some of the advice already given on previous posts, this will undoubtedly help your livestock and then take it from there when we have more information.
Don’t worry too much, we will help you as much as we can and if you’re unsure or if you get stuck with anything at all, just ask and we will be happy to help where we can