rain water vs. distilled water

[mete]

In a document (of Carolina biological supplies), about culturing or maintaining hydra, it writes: "Fill the containers almost to the top with springwater, filtered pond water, well water, or rain water. Do not use distilled water or tap water."

Why for example rain water is preferred over distilled water ? Because of the traces of minerals in rain water or is there something else that makes distilled water not preferrable ?

[MichaelG.]

On a larger scale : https://aquariumstoredepot.com/blogs/ne ... our%20fish.
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… so yes, I think your assumption is correct.
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Because of the traces of minerals in rain water

MichaelG

[Hobbyst46]

I would inquire with Carolina about this. Tap water can contain disinfecting chemicals, too much hardness, rust from plumbing, copper traces etc. So I can understand why it can be objectionable. However both rainwater and distilled water do not normally contain chemicals other than dissolved gases that make them slightly acidic.

[PeteM]

+1 to Hobbyst. In most areas of the US, a trace of chlorine is added to tap water. It's meant to kill precisely the kinds of things you'd want to view. So, that part (not using tap water) makes sense.

Distilled water is, essentially, rainwater. It evaporates and condenses. Unlike rainwater, it won't gather small contaminants -- possibly including bacteria -- as it condenses and falls. Why distilled water wouldn't be OK is a mystery to me unless some of those contaminants are wanted.

Store-bought water destined for uses like irons can also be treated by reverse osmosis. That method leaves a few more contaminants than distillation (minerals etc. that get past the membrane), but approaches the same level of purity.

[mete]

I asked this to the company. They said about minerals and rain being isotonic. Not sure if this also indicates it is buffered.

[Hobbyst46]

I asked this to the company. They said about minerals and rain being isotonic. Not sure if this also indicates it is buffered.

Apparently, what they mean with "rain water" is not pure rain drops collected during rainfall into an inert container, but freshly accumulated rain water on the ground - among rocks and in rock cavities, large leaves and flowers, sand pits etc.

[mete]

I asked this to the company. They said about minerals and rain being isotonic. Not sure if this also indicates it is buffered.

Apparently, what they mean with "rain water" is not pure rain drops collected during rainfall into an inert container, but freshly accumulated rain water on the ground - among rocks and in rock cavities, large leaves and flowers, sand pits etc.

My understanding (just learned after creating this post) is rain (water) is pretty different than distilled water. I didnt search much yet but this study for example comes up first in google:

"Rainwater is a mixed electrolyte that contains varying amounts of major and minor ions. Sodium, potassium, magnesium, calcium, chloride, bicarbonate, and sulfate ions are major constituents, to- gether with ammonia, nitrate, nitrite, nitrogen, and other nitroge- nous compounds (Hutchinson, 1957)."

https://pubs.usgs.gov/wsp/1535g/report.pdf

Basically anything in the atmosphere, also brought by winds etc., can be in the rain. I guess this also means it has some buffering capacity that prevents pH fluctuations to some extent. Also (unpolluted) rain water has pH ~5.6 not 7, due to CO2 in the air I read.

Also, I think, the company's answer for rain being isotonic is not correct, but I guess they meant it is closer to being isotonic than pure water. In other more correct wording it should be rain water is less hypotonic than distilled water. My initial thought was this difference would be unimportant but it seems not (but I didnt find more info about it yet).

[ddy5]

For quite a few years, we have used 'artificial pond water' for keeping crayfish. The high death rates we were getting when we used tap water disappeared. We weren't specifically considering protozoa, but there were often healthy clusters of peritrichs on the crayfish.

Artificial pond water might be more trouble than it's worth for the purposes here, but at least it would be a consistent, known composition. There might very well be similar solutions specifically for rearing hydra.
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The artificial pond water is easy to make from readily available chemicals: there are two stock solutions, and you add 20 ml. (4 teaspoons) of each solution to 5 liters of distilled water. (Add Solution A first, add some water, then add Solution B; if you mix A and B together, you can get a precipitate.)

Stock solution A: 175 gm NaCl, 35 gm CaCl2 (=46.3 gm CaCl2•2H20) in 2 liters of distilled water

Stock solution B: 15 gm NaHCO3 in 2 liters of distilled water

The stock solutions seem to be stable for long periods of time.

[ddy5]

I just found a methods paper from a lab that does research on Hydra. The solution they use to rear their Hydra is essentially identical to the artificial pond water we used for the crayfish.

Another lab uses a somewhat different solution, but the same idea. https://openhydra.org/wp-content/upload ... otocol.pdf

[mete]

For quite a few years, we have used 'artificial pond water' for keeping crayfish. The high death rates we were getting when we used tap water disappeared. We weren't specifically considering protozoa, but there were often healthy clusters of peritrichs on the crayfish.

Artificial pond water might be more trouble than it's worth for the purposes here, but at least it would be a consistent, known composition. There might very well be similar solutions specifically for rearing hydra.
__________________________________

The artificial pond water is easy to make from readily available chemicals: there are two stock solutions, and you add 20 ml. (4 teaspoons) of each solution to 5 liters of distilled water. (Add Solution A first, add some water, then add Solution B; if you mix A and B together, you can get a precipitate.)

Stock solution A: 175 gm NaCl, 35 gm CaCl2 (=46.3 gm CaCl2•2H20) in 2 liters of distilled water

Stock solution B: 15 gm NaHCO3 in 2 liters of distilled water

The stock solutions seem to be stable for long periods of time.

This is good to know, thanks for sharing.