The Puffer Forum

I would love to know the point that a brackish tank is considered marine. For all of the products to test water and chemicals that are labeled for freshwater use, and different products that say for salt water use. I would like to know when to switch over. Even my AP test kit has a different card for fresh and salt, but at what point do I switch cards?

Obviously my tank at 1.003 is not salt. Id like to know for future reference though.

It depends on the kit really. Some kits won't work properly at all with any salt, others will do alright at low salinity (1.005). Most kits that will work in Marine conditions will work just fine in fresh water though.

The nutrafin kits I use state clearly that they either work only in fresh water, or work in fresh and marine.

Marine conditions start around 1.020 or so, but that doesn't mean that a freshwater test kit will work up to that range.

The safest bet is to contact the maker of your kit and ask them directly.

As a rule of thumb, I always use medications that are suitable for BOTH freshwater and marine tanks when doing brackish. Even if there is only, say, 20% salinity, that salt could (potentially) interfere with the active chemicals in a freshwater-only medicene. A whitespot remedy that was 20% less effective would obviously be less good than a version that worked normally.

As for the threshold for marine, at SG 1.018 you can keep marine fish perfectly well, if not better than higher salinities. The story goes that the water contains more oxygen, puts less osmorgeulatory stress on the fish, and stresses parasites, which makes for happier, healthier fish. Inverts cannot be kept in this way because they are (mostly) osmoconformers rather than osmoregulators -- meaning that while fish can, within certain limits, maintain a fixed salinity inside their body regardless of what's going on outside, invertebrates usually are stuck with a salinity inside their body identical to that on the outside.

Cheers,

Neale

But test kits can be way off if intended for FW only and used in moderate to high BW, even if the chemistry is identical - the water of hydration makes life more complicated in testing and FW color charts may not work with significant minerals in the water.