you can use it for both. If you are using it to fill an new tank then just like any new tank you have to let it cycle. Cycling in the process where by good bacteria build up in the filter. these bacteria eat ammonia and break it down into nitrite. then more good bacteria come along and eat the nitrite and turn it into nitrate. which we then remove by doing regular water changes. this process can take up to a month.
once the tank is cycled then you can use HMA water to do you water changes as well. In fact it is really good for bristlenose as they like the soft water and the HMA filters will help in getting the right water conditions. Unlike RO water it wont remove all of the KH and GH, HMA filters are as the name suggests (Heavy Metal Axe) are designed to remove heavy metals as well as cholorine, and bacteria.
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