Toxicity and organ targeting freshwater fish

Tom Barr

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I am going to discuss a number of topics in this thread and continue along this same line of discussion.

One thing I've come to realize is that aquarist overall have a very poor understanding of fish biology. Sure, we know how to breed(perhaps plant folks much less so), feed, and keep groups together, but when it comes to envirponmental stresses, much less so.

This is not really our fault.

We have little research in the way of specific species, comparative studies and good understandings about the way dissolve gases influence fish health. I will be referring to Target Organ Toxicity in Marine and Freshwater Telelosts (Vol 1, Schlenk and Benson 2001)

The goal is to better understand the issues in aquariums about adding things like KNO3, CO2, O2 and current.

The Gill represents a multifunctional organ for fish.

Respiration
Ion regulation
Acid base regulation
Nitrogen waste excretion

It accounts for over 50% of the surface area, they provide a very thin layer of exchange between the water and the blood(about 0.5 to 10 um), and they have a massive flow over them, in part due to the low O2 capacity in water. This means that if the fish is exposed to a toxicant, the gills are typically where we will first see the effect/s. If you stop any of the above 4 main physiological processes, you have death, if you have sublethal exposure, then the quantification is much more difficult to access.

CO2 and O2.

These are exchanged at roughly equal molar concentrations at the gills.
with this concept in mind: think about high CO2 and high O2 together.
Now think about what happens if you have low O2 and high CO2.
High O2 and low CO2.

O2 moves through the water into the blood entirely by diffusion, and at least 90% of the CO2 moves via diffusion also in the opposite direction.
Since the partial pressures drives this diffusion, and since O2 is much less soluble than CO2 or NH4, the flow required by the gills is high to maintain enough O2.
This results in extremely low partial pressures of CO2 in the fish's blood(about 30X less than O2 typically).

The so called Borh and Root effects on fish: The addition of CO2 and /or H+ to the blood(like as is the case for us when we add CO2 gas) decreases the O2 affinity/capacity of hemoglobin. This effectively increases mean blood partial pressure of O2. So the fish hyperventilates to reduce and wash out excess CO2 by also increases the rate of O2 exchange across the gills.

So while the resting fish might be okay, when they really start swimming and exercising a lot, they are really in much worse shape. Sort of like us at high altitudes and trying to job or run at 12,000ft without any acclimatization. you suck wind and cannot catch your breath and cannot run.

CO2 excretion is less understood and more complex than O2 uptake in fish.
Buffering capacity(eg the KH) plays a large role here. So in general, without going into enzymatic pathways, the higher KH allows the CO2 to be excreted more rapidly and maintain a low CO2 internal partial pressure.

Who said hard water was bad?





Fish waste: about 70% is NH4 and the rest is urea.
 

Tom Barr

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CO2 gas is permeable to cell membranes, HCO3 is not.
Cells balance this by converting HCO3 to CO2 and exporting the waste CO2 via the gills. But this also gives insight to the reasons why adding KH rapidly can kill fish where as the CO2 does not.

http://www.awi.de/fileadmin/user_up...ysiology/_jpg_1024/PMT_Prj_Co2_Model_1024.jpg

this gives a good overview of what all is going on, follow the CO2 into the critter. You will see that the CO2 will be exchanged for HCO3, this in turn, is exchanged for Cl- in the transporter(HCO3-/Cl- transporter). You can consider the other pathways also in the diagram, but do not get too confused with it. We are focusing on the CO2/HCO3/Cl part and how it relates to CO2 additions and O2 levels in the aquarium.

Blood: a high CO2 ppm will lead to a lowering of blood pH. The fish responds by altering their HCO3 /CO2 ratio to optimize the pH. Such changes in CO2 ppms in our tanks occur rather slowly inside the fish. This is done mainly through changing the ion exchange processes at the gills and this also involves other ionregulation processes. the fish can lower the pH by adding acids like NH4 or H+'s or raise it by adding bases like OH, HCO3 to the blood. the two strongest counter ions with respect to acid base movement are Na+ and Cl-. These are involved the most in acid-base situations.
 

Tom Barr

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In general, the harder the water, the less toxic most chemicals will be.
Many aquarists assume that super soft RO water and frequent water changes are the best defense against Nitrogen waste

This is counter to fish and invertebrate physiology however, harder water (GH) will help mitigate toxic effects of NH4 and metals.

Regards,

Tom Barr
 

VaughnH

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Does this indicate that we need some minimum KH and GH for the benefit of the fish? I know plants don't care much about KH, but need the Ca and Mg from the GH, so they do care about it. How does this translate into ranges of KH and GH most suitable for planted tanks with fish?
 

Tom Barr

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I'm not so sure that there is some general minimum, however, more KH and Gh seem to help reduce toxicity a significant amount in virtually all fishes.


Regards,
Tom Barr
 

aquabillpers

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Tom Barr;26349 said:
I'm not so sure that there is some general minimum, however, more KH and Gh seem to help reduce toxicity a significant amount in virtually all fishes.

I've read that relatively benign ammonium becomes dangerous ammonia as the pH moves above 7.0. Since pH is often a function of KH, isn't there are conflict?

bill
 

MarmoteX

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Thank you Mr Tom, I was breaking my head with water parameters and mixing RO water with tap water. Here in Canary Islands we have very hard water and most people in the hobby have truble bringing down the hardeness

About ammonia and ammonium, the last one is the one escreted in urine, but as a weak acid it wil be a bad donor of protons. Although at a higher pH will increase the formation of ammonia, In a resonable pH interval for our aquarium, I would guess that it won´t mean much. Also you have to consider all of the ammonia removal by bacteria and plants.

From wikipedia (I don´t have time to calculate myself) at pH 7 99% of ammonia would protonate into ammonium.
 

Tom Barr

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aquabillpers;26377 said:
I've read that relatively benign ammonium becomes dangerous ammonia as the pH moves above 7.0. Since pH is often a function of KH, isn't there are conflict?

bill

To some degree and this is true for all heterogenous mixtures in solution(water).
We have some that act to counter the other, some that act synergistically, some antagonistically etc.

It can be a mess in real "field systems", vs the lab isolated situations.
Folks seldom make this distinction when applying research from the lab.

However, since some plant tanks add CO2, this might be worthwhile and not deal with higher pH's.

I think given that having some KH(50-100ppm), vs say, super soft water(5-40ppm)............might help and then not haver the issue with higher pH's.

There's a balance essentially.

no#2

There's a lot of plants and good bacteria, the NH4/NH3 really should not be an issue is a well run tank. Should not be much there.

Regards,
Tom Barr