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PostPosted: Jan 1st, '07, 19:01 
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Rupert - what I am thinking is experimenting with a char made from potasium rich plants. Just need to decide what to use - then I can do some experimentation which would include checking PH etc. I agree that the best option would be to add the stuff to the grow-bed for slow release into the system. I reckon char could serve this purpose???

Still need to find out though whether the system is actually deficient in potassium or whether it is just a side effect of salt in the water. Does anybody know how I would use my beer hydrometer to work out the salt concentration of my tank. I guess I could just make up a solution of known concentration and check the sg of that. Answered my own question.


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PostPosted: Jan 1st, '07, 20:46 
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VB, according to my BIL you can use a standard multimeter to check salinity levels. Just hold the probes the right distance apart (I don't know what distance that is) TYhen test rainwater then test tap water and see the readings. Have no idea what I am talking about. But could be worth further investigation


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PostPosted: Jan 1st, '07, 21:05 
VB, another idea (i mentioned this in another post... ) was to use some "unwashed" cuttlefish shells in the system.

Mainly for buffering but work on the principle (maybe totally wrong) that they may have absorbed some trace elements from the sea water.

i.e

Dissolved salts in
sea water (atoms):
55.3 % Chlorine
30.8 % Sodium
3.7 % Magnesium
2.6 % Sulfur
1.2 % Calcium
1.1 % Potassium

Another thought was maybe some seaweed or "crushed" algae/seaweed.

I've used seaweed for years on the garden, usually at preparation stage, without washing it (was told to, but couldn't be bothered) both dug in and just layered on the surface.

Found it conditioned soil really well and any leaf or fruit crops excelled.

Baring in mind the effect of salinity on potassium take up I think for AP it would have to be washed.


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PostPosted: Jan 1st, '07, 21:39 
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Yes a multimeter will work using the resistance function. Just get the probes the same distance apart for each test. The more the dissolved salts the lower the resistance... Test using some actual sea water if possible. If not mix up known mixtures of salts. NaCl (Table Salt) + MgSO4 (Epsom Salts) + NaHCO3 (Bicarb of Soda)... You can then roughly get an approximation of how the different salts effect salinity.


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PostPosted: Jan 1st, '07, 22:08 
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I've often wondered if by changing the diet of the animal whether the nutrient mix of the manure can be altered. Bananas, spinach and wheat-germ are among the potassium-rich foods, and if these were made available to the fish in some form acceptable to them, would this increase the K content of the manure ?

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Sardines, pilchards and potato crisps are also high in K - could be some interesting mixes of fish food coming out of my blender soon !

Spinach could be the first experiment - just chuck it in the pond and see how they go for it.


I was having a look in some medical books tonight (trying to stay awake on night shift). In a healthy human, 80-90% of excess potassium will be excreeted via the kidneys, the rest via faeces or sweat. If you have a high potasium diet, what you dont need will end up in the toilet (or on the lemon tree if youre that way inclined). I'm not sure if fish have kidneys or not but they will excreet it in one way or another.
Other high potassium foods include: artichokes, broccoli, brussel sprouts, cabbage, carrots, celery, collards, cucumber, mushrooms, potatoes with skins, spinach, tomatoes, apricots, bananas, cantaloupe, guava, honeydew mellon, nectarine, orange, prunes, strawberries, tangerine, watermellon, brewed coffee, and of course baked beans. I'm not sure which of these fish will eat but I think you're on the right track Mick. I also suspect Steve is right when he said dont remove the solid waste at the bottom of your tank. I guess also that the above mentioned fruit&veg are the ones that would need a relatively high potassium level to grow.


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PostPosted: Jan 2nd, '07, 01:01 
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A combination of potassium and calcium is used to buffer pH and provide the other essential plant nutrients. We also add a little chelated iron, as fish food is lacking in iron and the plants require it to produce chlorophyll. That is all we add to the system - fish food, a little buffer each day, and a little iron once a week. All the micronutrients required for the plants are in the fish food, so we don’t need to add any of these.

This comes from an article about Wilson Lennard's research found here:
http://www.hydroponics.com.au/back_issues/issue83.html
I wonder what sort of potassium (and calcium) he used?


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PostPosted: Jan 2nd, '07, 02:53 
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Here the CSIRO have been using potash in aquaculture
http://www.publish.csiro.au/?act=view_f ... 125p33.pdf
"when we add potassium,
in the form of potash, to boost concentrations
to at least 40% that of seawater, the
survival and growth of marine fish and
prawns pleasingly match those in seawater.’
Two freshwater species, silver perch and
rainbow trout, flourish in the saline
groundwater even without the addition of
potassium."


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PostPosted: Jan 2nd, '07, 06:07 
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I think the conductance is proportional to the number of valence electrons * concentration *wire area / distance between them. Fix the distance and you'll basically measure the relative number of -ves in the solution. Sodium (Na<sup>+</sup>) has one, Calcium (Ca<sup>2+</sup>) has two.

So if you can calibrate from a known concentration (RO water + a known amount of salt) you can measure any liquid. It is probably worth making a simple probe using some chrome plated metal attached to a block of plastic or wood. One issue is that continuous measurement will tend to polarise the probes so they act like a battery and give erroneous results. Commercial probes use AC instead.


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PostPosted: Jan 2nd, '07, 07:34 
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One issue is that continuous measurement will tend to polarise the probes so they act like a battery


If you use stainless probes of the same metal and clean them after use they should be OK... I don't think a multimeter would have enough current to electro plate the probes in such a short time. Also keep the copper leads out of the water...


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PostPosted: Jan 2nd, '07, 08:01 
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Stainless steel relies on the passivation from chrome oxide. But this is a) not conductive and b) corrodes electrochemically. I think stainless might lead to some trouble. Better would be gold. electrolysis of water requires about 1V from memory, so I guess if we use a lower voltage no electrolysis would occur.


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PostPosted: Jan 2nd, '07, 09:23 
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All my electrolysis devices/hydrogen generators have used #316 stainless, and they perform very well. Conductivity is not an issue, not in this application anyway.

Graphite/Carbon is another cheap alternative to inert electrodes, use two graphite pencils... Gold may be a bit costly for this application


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PostPosted: Jan 2nd, '07, 09:27 
Seriously, this cant be healthy.
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beat me to the punch there TC. another probe could be platinum that also way exy.
Use to extract silver from photographic waste.
using carbon anodes and 316 ss for cathodes to collect the silver, the SS would pit out after a while.


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PostPosted: Jan 2nd, '07, 13:03 
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I could see my wife now. "you bught gold for your fish and gave me this stupid...." fill in the blank. I think i would stick to some graphite pencils on that one :D


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PostPosted: Jan 2nd, '07, 13:59 
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Yeah, graphite is excellent. Forgot about that.


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PostPosted: Jan 2nd, '07, 17:56 
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Thanks guys - but I might just use a hydrometer.


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