May sound stupid, but is there a way to get the calcium out of it ? It is going to be in a tank with neon tetras and zebra danios, and a catfish.
Not a stupid question, but no, it's literally what the rock is made of. As mentioned, it will increase your pH, which would not be appreciated by Tetras and catfishes.
And just to be clear, the fishes you mentioned you'd like to keep (Tetras, catfish, Danios) are all warm-water, Tropical fish, and will need a heater. You want the tank about 25-26C for these fish.
Ah ok thank you, also for the filter, should I just use sponges, or should I use calcium carbonate aswell ?
You don't want calcium carbonate in a tank with those fish, it will raise your pH. You can do just sponges, you can use plastic bio balls, I use plastic kitchen pot scrubbies from my local Dollar store, maybe you have something similar in your area?
