Backyard Aquaponics
A place to discuss aquaponics

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.

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

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.

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.

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.

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?

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."

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⁺) has one, Calcium (Ca²⁺) 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.

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...

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.

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

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.

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

Yeah, graphite is excellent. Forgot about that.

Thanks guys - but I might just use a hydrometer.