Q: How do you cope with the potassium deficiency in these systems?

Note:
The transformation of fish ammonia occurs in the nitrification process:

Optimisation of the nitrification process requires an understanding of the biological processes and organisms involved. The conversion of ammonia to nitrate is a two-step process carried out by two groups of bacteria belonging to the genus Nitrosomonas and Nitrobacter.
Neither of these bacteria can metabolize organic carbon to meet their energy requirements. Instead they derive their energy for growth from the oxidation of inorganic nitrogen compounds such as ammonia and nitrite.
As a result of their unique metabolic pathways their growth rates are extremely slow and typical doubling times are in days compared to less than an hour for organisms that metabolise organic carbon. The slow growth rates of the nitrifying bacteria make them very sensitive to a wide range of physical and chemical parameters. Consequently a thorough understanding of the operational parameters and regular monitoring of the process is vital to ensure maximum nitrification is achieved. Optimal nitrification occurs when the temperature is about 30C, the levels of dissolved oxygen and available alkalinity are high, the pH is about 8.0, salinity is low, BOD levels are low, heavy metals are absent and biomass is attached to solid media, there is sufficient retention time and adequate mixing. The key parameters to monitor are temperature, pH, ammonia and nitrate levels. If any problems are noticed with the nitrification process then additional monitoring needs to be done on the dissolved oxygen level, available alkalinity level and BOD levels. If serious problems with nitrification are still encountered more involved monitoring is needed to find out the chemical composition of the effluent.
1. Ensure that the aeration system is providing sufficient oxygen for both the fish and the nitrifying bacteria. The latter consume oxygen when they convert the ammonia and nitrite to nitrates. Overdosing a pond or aquarium with nitrifiers is not a problem as the amount of oxygen consumed by the bacteria is dependant on the amount of ammonia and nitrite present in the water and not the number of bacteria present. The oxygen level ideally should be maintained at levels above 2ppm.
2. Ensure that the fish are not being overfed.
3. Check that the pH level is close to neutral. The nitrification process will slow down when pH values drop below 6 and go over 9.
4. Check the temperature. Nitrifying bacteria perform best between 18°C - 30°C. When the fish are dormant in cold weather ammonia will not be excreted.
5. The nitrification process occurs best in environments with a low BOD level. Decaying organic matter is the usual cause of BOD in ponds. BOD should not be allowed to go above 200mg/L. Levels below 50 mg/L are ideal.
6. The nitrification process consumes a large amount of alkalinity. Ideally the alkalinity needs to be greater than 200 units (measured as calcium carbonate).

By far the best filters for this purpose are the socalled "trickle filters".

Most of these filters are loaded with socalled "bioballs". They are black, 1.5" diameter, high density polyethylene.
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A good presentation on these filters can be found here:
Trickle filters

Hi
WOW just the questions I was going to ask !! lol
I recently set up a system in the ballpark of those described but for different reasons.
Will try to keep this brief so if you need more details??
I built what is called a "seep wall" a low volume waterfall that is used to grow epiphytes . I use a gallon
aquarium as the reservoir.This is also connected to a thousand gallon lily pool. So the water is released at the top of the "falls" goes down the wall into a collection area which serves as a marsh gardemn into the aquarium overflows into the pool . i keep fish in both the aquarium and pool as well as plants. Obviously as well as being a great grow area I'm getting some kind of hydroponic system
How can I test to see how effective it is and what should I test for?? Thanks gary

The biggest problem with these systems is its capacity to transform the highly toxic fish urine (mostly ammonia) into less harmful nitrates.
Ammonia can be lethal at levels around 0.5ppm.

Get an ammonia test kit from a fish shop. Preferably the drop tests. Test once a week at first. A good system will keep the ammonia levels close to zero. If not . . . then replace some of the water immediately when it gets close to 0.5ppm. In that case you will have to remove some of the fish and incorporate an effective nitrification system. Beware . . such a system may take up to two month before it becomes effective.

If you want to know if your plants are effective . . get a (similar) nitrate test kit. If the nitrates crawl up (above 20ppm) add more plants or replace some of the water.

Algae risks can be minimized by checking the ratio between nitrates and phosphates. It should be around 10:1
So an additional phosphate kit might come in handy.
High phosphate levels are usually a result of excess feeding and could cause the growth of toxic blue(cyano-)algae.
High nitrate levels cause a bloom of green algae.

hi
Have a lot of experience with fish systems and fish and plant systems . What i have done here is combine this with a watering system for an epiphytes area .
The usual management for these type plants is frequent weak ferts applied in liquid form.. Obviously as a byproduct of watering I'm getting some type of feeding.
What i need is some way to establish just how much is left over from the water garden?? I suspect that it is very weak so therefore would have to be supplemented in the usual way.
Ammonia test would be useless as the fish would be dead if it was present.lol Same is true of nitrite
Nitrate might be useful but since the 3 systems are all using the same water. ?? The water and marsh area are both doing well so see no need for testing there but need to arrive at a way to tell how effectively the system is feeding the epiphytes.
i feel a foliar feeding would be best but naturally the excess would end up in the water area. Overfeeding would be a serious problem for all three systems.
All the epiphytes are doing well so i feel that the "hydroponics" in the system is working far more effectively than I thought. But how to know?? thanks for the help gary

Not sure if I can help.
Epiphytes are known to use nutrients sparingly. So, even if they grow well, your system may still nitrificate inefficiently.
In your place, I would still use a ammonia/nitrate/phosphate testkit and see what conclusions I can take from the results.

Hi
The system has been setup two years in March with the seep wall added last summer.. It is made of expanding foam
Both the aquarium and the pool have done very well but had to redo the pool this spring due to invasive growth .Put in a new liner while doing it . Both the fish and the plants have done very well. In fact a bit too much lol The vines ate through the fiber glass wall liner and caused some leaks, so can't run the system 24/7. But still serves as built it watering system.
The whole system contains 1200 gallons. and last winter was the first attempt to keep it warm through winter. Worked well but did not heat the air as I'd hoped.
Anyway. My main concern now is for the epiphytes. on the wall. though all have done very well with some minor light problems particularly with the orchids but may be too soon to tell. All were small offsets and probably not good choices for this type of mount.
Trying to decide if I should add regular ferts even though I'm getting some from the water system through the summer grow season. have always followed a "weakly weekly program
but of course now the excess will drain into the water systems.
I checked the ammonia, nitrite both at zero but you could just look at the fish and know that lol. Don't have a nitrate test. but I'm always suspicious of the cheaper test kits. But you can look at the aquarium plants and know they must be getting most of what they need. I'm pruning once a week to keep them in check.lol
I guess for the epi's I'll hold off on the ferts until I see a problem. Thanks for the help gary