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Breeding cardinal tetras - transitioning from very hard to soft water?

xZaiox

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31 Mar 2022
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Maidstone, UK
Hi guys,

My tap water is hard - roughly dKH of 10 and dGH of 20. I have about 19 cardinal tetras in my main tank, and due to my water hardness, I had just pretty much ruled out ever breeding them. I don't have an RO/DI system either. However, I then read a post about breeding them in rainwater... I don't know why I had never considered rainwater, how simple!

From my understanding, these fish can be bred by just feeding them well and then putting a mature male and female into a separate tank with something like java moss to scatter the eggs in, and leaving the lights off. This all sounds fine, but how do I go about transitioning the fish from my main tank with hard water to the spare one with rainwater? I would have thought a jump in parameters such as this would be much rougher on the fish than a standard acclimation from shop to tank? Or is it pretty much the same thing?

Does anyone have any advice regarding this? Would also love to hear from anybody who has bred these fish.
 

brhau

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10 Jul 2020
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10% water changes a day should be slow enough. You could choose to only do it in the breeding tank, but tetras would appreciate it in the main tank as well.

You could also drip acclimate, which requires more of your attention. Just make sure to do it in both directions if you choose to have different water parameters across the two tanks.
 

Conort2

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10-20 percent at a time would be good. I’d go for much larger changes in one go if I’m honest but that’s just me, I’m of the belief fish can take substantial changes as they would in the wild.

However do 10-20 percent to be on the safe side!

Cheers
 

tam

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5 May 2011
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A TDS pen might be helpful to you to monitor the change. Have a read up on osmotic shock - that's the issue you encounter moving fish from one hardness to another rapidly. I'd also suggest considering lowering the hardness in your main tank - even if you mix that to 50/50 tap/rain it would give you a lower starting point. Otherwise, pop them in a seperate tank with the tap water and gradually change in out for rain over a couple of weeks. I'm guessing you feed them up for a bit anyway. Keep in mind you'll need to gradually transition them back too.
 

castle

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19 Dec 2015
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norfolk
Does anyone have any advice regarding this? Would also love to hear from anybody who has bred these fish.

Going back in time here, 2004-6ish.
I had a darkwater setup (twigs, leaves, elodea), I was taking home 5L of water from a stream I'd cycle past every most day(s). And I fed the fish purely on bugs from the village pond.
They'll spawn happily once they're in condition. But, moving them to a tank with a lower hardness is key (imo) as that fresh, softer water I guess feels like rain to them? I dunno. They'll spawn every day, and eat their own eggs... once in condition I would split 6 pairs across 25l tanks, leave them in there for the night, check in the morning to see if there are eggs, if eggs, remove. Repeat this cycle every 4 weeks or so.

Start some bbs.
Have green water ready, boil a bit of cabbage, put it (cooled) with some tank water in a large mason jar in the conservatory. It'll stink, it'll work.

Somethings to note:
Stuck fish is a thing, so eggs are stored and less fertile, so expect a few bad lays before new eggs are produced.
Egg sacks last about ~3 days, then you must feed green water.
Water cleanliness at this stage is super important, small wc are needed - but only if you're using egg yolk powder to feed, which I don't recommend if you got green water.
You could probably get around +30 fry a spawn.

Oh, and sex identification iirc; males have a more warped lateral stripe, females is straighter. Females from above are more plump.
Not worth much. Try and find wild caught. Absolutely be picky about the Tetra you spawn, no bad spines, finnage etc.
 

xZaiox

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Maidstone, UK
A TDS pen might be helpful to you to monitor the change.
Good tip, thanks 😁
Have a read up on osmotic shock - that's the issue you encounter moving fish from one hardness to another rapidly.
Yeah this is what I'm apprehensive of. I've never worked with soft water before and since rainwater will be very soft, I wasn't sure how stressful it will be for fish to go through such a large change.
I’d go for much larger changes in one go if I’m honest but that’s just me, I’m of the belief fish can take substantial changes as they would in the wild.
What sort of percentage changes would you personally do? I tend to have the same opinion, and I wouldn't hesitate to quickly transition a fish from moderate hardness to very hard water, but since this is a very large change I wasn't sure just how cautious I should be or how quickly fish adjust.
@castle - all of this information is really helpful to me, thank you! :thumbup:
 

brhau

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10 Jul 2020
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San Francisco, CA, USA
Going from high osmolarity to low osmolarity is definitely the more dangerous direction. It's impossible to say what percentages are guaranteed to be safe, but it definitely helps to know the differential between your two waters.

I'm definitely on the conservative side, but here's my experience:
  • I've taken a tank from 500 TDS to 100 TDS by doing 10% daily changes with 0 TDS water with no ill effects. Could it be increased to 20%? Probably, but I wasn't in a rush.
  • I've drip acclimated fish within a day from 1700 TDS bag water to 20 TDS tank water. It took all day, however. I transferred the fish when the TDS got below 64 and the pH was within about 0.5 units. My rationale was that the difference between 64 TDS and 20 TDS might not be much different from a heavy rainfall on a shallow swamp. That said, the absolute osmolarity is already quite low, and it's a relatively small body of water. I doubt tetras would see TDS in the hundreds in nature, and then see it quickly drop to the teens.
-Ben
 

Conort2

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What sort of percentage changes would you personally do?
I’d go 50 percent if it was me however osmotic shock is definitely a thing although I’ve never experienced it. I’ll often change 50percent almost straight RO which is a few degrees cooler to try and trigger my corydoras to spawn.

In the Amazon when the rains arrive the tds, temperature and chemistry can change rapidly and small dried out stream can turn into raging torrents in no time. These changes are what certain fish need to spawn.
 

Conort2

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I doubt tetras would see TDS in the hundreds in nature, and then see it quickly drop to the teens.
This is a good point. If you’re in no rush it certainly won’t hurt to take your time.

I’d also recommend dropping your KH/tds in the main tank. I know people say cardinals are ok in harder water but I’m of the belief you should try keep fish as close to what they’d expect in nature.
 

brhau

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Location
San Francisco, CA, USA
I’d go 50 percent if it was me however osmotic shock is definitely a thing although I’ve never experienced it. I’ll often change 50percent almost straight RO which is a few degrees cooler to try and trigger my corydoras to spawn.
Here's where it helps to know the EC/TDS. I have no problem with 50% if the water in the tank is already relatively soft. If it's very hard, I would not do a 50% change with straight RO.

I know people say cardinals are ok in harder water but I’m of the belief you should try keep fish as close to what they’d expect in nature.
100% agree. Even if the cardinals are apparently fine, they could suffer long term damage from needing to regulate a higher osmotic gradient that they've been evolved to do. It's an energy expense that needs to come from somewhere.

-B
 
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