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i have a small aquaponics system running in our test hydroponic greenhouse, the tank is only 300 litres with a flood n drain grow bed above about half the size with a bell siphon installed on a constant cycle. we have very soft water around here and the kH has always been an issue but didnt seem to affect things before. in the last few weeks things have changed we have lost several fish and the water chemisty seems all over the place, the ph was dropping rapidly and the ammonia is rising. i did a half tank change last week with some harder water which raised the kH and i also hung a bag of oyster shells to help keep this stable, however the ph is dropping again and the kH seems to be back to close to zero. and today we have two more deaths.

these are my current readings:

nitrites 0.25-0.5

kH 17ppm

pH 6.0-6.2

ammonia - 0.5

temperature is about 75F

i understand that the low kH is not buffering the pH which is most likely messing about with the conversion of ammonia to nitrites and onto nitrates. but why does the kH drop so much?

when i changed the tank with harder water the kH came out around 100ppm but within a week this has disappeared even with the oyster shells added. so i know the ammonia is too high and the chemisty constantly fluctuating could be the main cause for the fish deaths.

what should i do to fix this problem? travveling to a deifferent water source to fill up containers of hard water doesnt really do much for our carbon footprint. 

i am planning on doing this tomorrow as a friend is bringing me some water with a better kH but i need to top up with our soft water usually so whats the best way to keep the kH up and stop the ph shifting so much?

am i close or am i way off the mark?

help before we have none left.

the only other thing is a lot of the tilapia do not seem to of grown much whilst others have grown a lot and i have noticed some of the larger fish being bullies to the smaller ones, its mainly small fish that have died. but they are all off their food almost completely, apart from one or two of the largest fish. someone mentioned some may have turned female and are getting bullied by the males getting a bit frisky but i dont personally think this is the problem.

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2 options (i have heard so far) to maintain a "friendly tank".  The problem is that the dominants will force the minor males to the top areas (where they cannot breed with the females), and harrass the females in their attempts to breed.

1) maintain a "male only" tank - so that the dominant male/s will not need to go into aggresive breeding behaviour, (and even females try to maintain/defend a territory when they are breeding)


2) maintain a higher density of tilapia (of relatively same size) - I have a 75Gallon "community tank" with around 50 mostly 5-inch fingerlings, and the two dominant males controlled both ends of the tank, allowing the females and minor males to stay in the middle area of the tank. When one of the dominant males tries to go to the middle, it usually ends up fighting with the other (leaving the rest to scamper off elsewhere). Any other "bullying" gets spread around the other tilapia, and they can easily hide among ttheir brethrens.

Just remembered, have you put caves for the females to hide in? a length of 4" pipe (or a size big enough for the females but not for the dominant male/s) for each of your females should do. 



Ian Hawkins said:

thats worrying as i dont have anywhere else for them to go and i was just going to keep stocking and removing the larger fish as they grow. is this not really viable?

Joel G said:


In commercial tilapia ponds, it is recommended that the fishes be roughly of the same size to prevent "bullying". Separating by size gives the smaller tilapia equal chances to eat and are less stressed. Fingerlings are even segregated into up to 10 different sizes. 


Mature tilapia are even worse bullies.


i was sold these tilapia as all male and they were all roughly 5-8cm when i got them, now some are 8-10cm some are more like 15-20cm which is weird why some have grown and some not. they still dont want to eat and im really worried they are going to die from starvation. 

The ammonia is now minimal, o nitrites, and the kH has gone up to 70-80 not ideal but an improvement so im still raising it gradually, pH seems to have stabilised and stays around 6.8 now so all looks fine on the tests. but they still arent eating so no nutrients for plants and as a result. i have tested the nitrate (next to zero) and phosphate also nearly nothing.


things arent looking good. 

What kind of feed are you using?

Sounds like they have all grown so they are not dieing of starvation.  Some fish are eager to come up and eat no matter what but I've also had fish that were shy about eating while I was there.

Unless they are penny fish, the all male has some drawbacks.

Now just because they were sold as all male, that doesn't mean they are all genetically male if they were made all male using hormones in their food as fry, then some of them are still genetically female and though they will still grow faster than if they had developed female, they will probably still be smaller than the true males.  If they were sorted manually to give you all males, then I fear they were probably actually too small to make sorting all that accurate and there could be some females in the bunch.

And even with all males, there will still be size variation and growth rate variation among them.

im not sure about the feed i was given a big bag from the breeder when i got them and have just used that ever since, they used to eat almost out of my hand and would bite your finger when they were hungry but now they barely come up, they really have hardly eaten anything for a few weeks now. im considering getting another tank and linking both together feeding the one bed, that way i can segregate the smaller fish. although this will probably mean not enough bed for the amount of water. i dont have the space to expand really and add new beds.


just thought i could try and split the tank in two with some mesh or similar and segregate them that way?

Yep you can do a tank divider or make a cage for one group or the other.

As you get your alkalinity and carbonate hardness up a bit the fish should become more interested in food.  However, keep in mind as tilapia mature, they gain the ability to filter feed.  The younger juveniles don't yet have this ability and are super hungry with an intense protein hunger (hence why they would nibble your fingers since that kind of hunger trumps fear and shyness) and as they get older they may just be filter feeding and getting enough of what they need and not be as hungry for the pellets. 

im pretty sure its a water isuue now i have had another death and the kH dropped again and i have ammonia up at 1.0 now, i think im going to have to do a partial tan change again, but i only have really soft water with virtually no kH here, im going to have to raise it before it goes into the tank i think. fingers crossed i can stop any more deaths.

What pH did it drop to?  I know of some one who had some real bad problems when they switched to rain water and I think they found they not only needed lime and potassium bicarbonate but I think they also wound up having to soak some marl in the water.  I'm not entirely sure what that is or where to get it but their system eventually stabilized.

the pH still seems to be fairly stable its at about 6.6 ish. hmmm never heard of marl ill look into it.

ok so after a long struggle i now have no tilapia left. i have had a control over the levels for a while now but still the deaths kept occuring, so now they have all gone and im waning to try again but i dont know what my options are. on a different topic i mentioned the problems which our supplier had told us about with the hydroton pebbles and before i wanted to get any more fish in i thought id get the water analysed as we send off samples weekly from our lab anyway. im not sure about the tolerance of tilapia to much apart from whats on the normal test kits so i'm asking for a little help.

calciums up at 148mg/l

chloride 0.6mg/l

sulphate as SO4 337mg/l - this is high where has all this come from?

nickel is at 16.4ug/l this doesnt seem very high but where has it come from?

aluminium 7.2ug/l again low but where does the aluminum come from?

the full results are attached if someone knowledgeable would care to take a look id be ever so grateful. 

at the moment my system is running fishless and i dont want to restock fish into bad water.


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