Ultra low pH. How safe is it, really?
My tap water degases down to 6.2 rapidly and then proceeds to drop below testable levels within days. I've been using crushed coral for years but have lately been wondering if I've been wasting my time. My tap water has a Total hardness (whatever that means) of 11.3 and a Calcium hardness of 8.9. I don't know of I should add those numbers together to calculate my GH or if the Total includes the Calcium hardness. If my GH is less than 1, should I add epsom salt to bring it up a little for safety's sake and get rid of the crushed coral at last? I've always felt dumb using it as a buffer and since I've learned that the degas process isn't as terrible as I thought it was (using pH alone as a guide is not as reliable as hardness!), I'm totally ready to let my water drop into the -6.0 range if it means I can grow plants better, breed oddball amazon species etc. So please let me know how this all works. I don't have plant soil or anything fancy like expensive lights and I don't expect to grow difficult plants, etc. I just want to get some insight on what I'm about to put my fish and plants through.
it mostly depends on your plants and fish.
Calcium works a little, but I prefer crushed limestone. I'd be willing to bet that adding a tsp per gallon directly to the substrate wouldn't even raise the ph, but would stabilise it to stay around 6.2-6.5.
Many plants get their calcium and magnesium from the water, so with ultra soft it may be lacking (even with liquid fertilisers).
The limestone is cheap at a gardening center (I got mine for $4 for 5Ib's, and I only use a tbsp or two per year.)
The main issue is the fish. Plants are generally more adaptable, and much less likely to be seriously affected (= killed) by low pH.
Presumably you are dealing with soft water fish such as are native to most of the Amazon basin, SE Asia, and West-central Africa. Within these areas though, there are variances in pH and to a much lesser extent hardness. Fish endemic to this or that river system are naturally going to do better with the aquarium water in close to the same range, though there is quite a bit of adaptability here too. And wild-caught fish are one thing, tank-raised fish (for several generations) are another, even sometimes with the same species.
My tap water has <1 dGH and KH. With so little, almost zero, hardness, the pH in the aquaria can lower below 5 easily, and do. But I have wild-caught fish from comparable waters. And my plants certainly grow well.
Fish like plants also need calcium, and other minerals, and they obtain it from the water and from food. It is no surprise that a major food source for most Amazonian and SE Asian fish are crustaceans. It is interesting to wonder how crustaceans manage their exoskeletons (which need calcium) in such soft water. I can't answer that one.
The problem with allowing such low pH in aquaria is that it is a closed system. In their natural habitats, fish are not confined anywhere as closely as they are in even the largest aquarium, and water in nature is continually being changed. Diurnal temperature changes of several degrees are common, as are significant pH changes caused by several factors, including but not limited to rain. The partial water change is crucial to attempt to replicate this condition, and that is one of the best ways to regulate a stable pH.
I have used dolomite to prevent the pH from dropping below 6, and again depending upon the fish this may be advisable. Dolomite releases calcium and magnesium into the water, and a little goes a long way.
I hope the above generalities help a little. If you want to discuss further, perhaps you could provide the fish species (and if they are wild-caught, if you know that) and plants too, as there are a few that will not fare as well in very soft acidic water.
The species in question (that I would keep at the lowest pH long-term) are my angelfish, jurupari, brachyrhamdia meesi, platysilurus malarmo/mucosus (wild-caught), acanthicus adonis, my doradid collection (mostly wild caught) and a few others like my Oscar (wild caught from Peru), etc. I would keep a buffered tank for the others like my mosquitofish, mollies, jade goby, hoplo cats etc.
Tetra make a low pH test kit that goes down to 5, and I have used this. Too low a pH can cause trouble for some species; Dr. Stanley Weitzman pointed this out in an article in TFH some years ago, with observations from his own captive (wild-caught) fish. Too low a pH can be as troublesome as too high a pH since pH directly affects a fish's metabolism.
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