Water change/pH Question

[jason the filter freak]

On a nano or pico sub 7 gal range, with a weekly or bi weekly water change of 10-25% of water volume, how nessecary would it be to use Kalk to do daily or bi-daily top off (depending on evap rate) I would test this and find out myself but I don't have a decent pH test kit.

 

Can any one shed some light here?

[fab]

Evaporation rate, measured in inches of water depth per day should not be very different between a nano, a pico or a mega tank given similar relative humidity, lighting and heat intensity at the water surface. The problem is that the percentage of the tank's volume that evaporates will be highest in a pico, because of its shallowness.

 

The percentage of tank volume evaporated is the ratio of the number inches of the depth of evaporation to the tank depth, or:

 

% reduction in volume = inches lost to evaporation / tank depth in inches.

 

So,

one inch of evaporation in a 25" deep tank is 1/25 = 4% reduction in water volume.

Whereas,

one inch of evaporation in a 10" deep tank is 1/10 = 10% reduction in water volume.

 

Fact: Seawater runs between 33 and 37 parts per thousand - globally and all depths.

 

Seawater is nominally 3.5% salt. That equates to 35 parts per thousand (ppt), or a specific gravity of 1.026.

Specifically, 41.8 grams of salt per liter of fresh water produces 35 ppt saltwater.

 

If you lose 10% of the volume of a tank of seawater to evaporation, you will end up with 90% of the water volume and 100% of the orginal salt content still in the tank. That runs up the salt as a percentage of water volume to 3.89%, or 38.9 ppt.

 

In terms of specific gravity:

 

seawater at 35.0 ppt = 1.026 specific gravity. By losing 10% of water volume to evaporation, the tank

evaporates to 38.9 ppt = 1.029 specific gravity.

 

In the case of the deeper tank that only loses 4% of its volume for an inch of evaporative loss in depth:

the salt content would end up at 3.65% or 36.5 ppt.

 

evaporation to 36.5 ppt = 1.027 specific gravity.

 

The shallower tank that loses 10% due to evaporation exceeds the saltiest water in all the oceans by a lot, 38.9 ppt versus 37 ppt.

 

The deeper tank that loses 4% due to evaporation remains inside the maximum ocean salinity, 36.5 ppt versus 37 ppt.

 

I'd say it is very important not to let the tank depth be reduced much at all by evaporation in a shallow tank. Depending on where your livestock comes from, you might need to be concerned about even 2% depth loss, which would end up at 35.7 ppt which is a specific gravity of about 1.0266 from a starting point of 1.026 as I have presented the data.

 

 

As points of reference:

Indo Pacific water salinity is sometimes relatively low, specific gravity 1.022-1.026

The Red Sea is more saline, specific gravity 1.027-1.028.

That is why specific gravities of 1.025-1.026 are considered fairly ideal for saltwater aquariums as it hits right at the near-overlap in these two ranges.

 

And by extension:

Shallow breeder tanks or shallow coral growout raceways suffer the same problem as discussed above. If one tank loses one inch of water depth in a period of time, other tanks near it should also be expected to lose an inch of depth. It doesn't matter whether a tank is 10 gallons or 1000 gallons. So the shallower tanks will be likely to have more serious concentration increases than deeper tanks. The concentration increases are for all material in solution, not just salt.

 

fab

Edited July 1, 2007 by fab

[Guest NSC]

Excellent response on specific gravity! But I think with a weekly 10%-20% water change your calcium and alk should be held pretty good depending on the bioload.

[fab]

I did not try to address Jason's question of calcium and kalkwasser dosing. Water changes exchange saltwater for saltwater with calcium additives. Clearly that can upset the chemistry of alkalinity and the buffering capacity in a system if those concentration issues are not tended to properly when a water change is performed.

 

Instead, I picked up on an issue that Jason did not ask about, explicitly that is how evaporation affects salinity, which turns out to be an important issue for how often to makeup fresh water in small tanks. My caution here is to recognize the sensitivity of shallow systems to evaporation that can show up as larger swings in salinity than might be expected. I also caution that large, shallow tanks have the same problem. The problem I discuss in the prior post ties to the depth of a tank, not its volume.

 

fab

Edited July 1, 2007 by fab

[Guest NSC]

Don't get me wrong, as I mentioned that was a very comprehensive response, very informitive and I am already plotting on a refractometer(don't have one). My point being depending on salt mix(quality) and the frequency of water changes the other levels should, in theory remain stable depending on load, evap. and so on.

[fab]

No offense taken. I just wanted to clarify that I was not responding directly to the specific question Jason had asked.

 

WRT the water change issue and the calcium kalkwasser aspect of the issue ...

If you have a certain level of calcium concentration/buffering in your tank and you do a water exchange, the calcium concentration will change unless you load up the new water with the right amount to yield the

desired concentration after the exchange. If the concentration is at the level you want it to be before the exchange, then you just have to match it with the new water, presuming you are replacing saltwater in the tank with the same amount of new water.

 

fab

[toastiireefs]

too much information to absorb all at once

[treesprite]

i found it quite educational, and it encourages me to be more diligent about topoffs in the winter when evap is high from the dry air

[jason the filter freak]

I see your point about evap, however due to my unfortunate circumstance you'd be wrong about tanks loosing the same amount of water... my 55 standard dimensions 48 1/4 x 12 3/4 x 21 where as the pico in question 2.5 is 12 3/16 x 6 1/8 x 8 1/8

 

Given the amount of rock and other displacing objects in each tank I'd say the 2.5 is holding about 2.1 gal of water and the 55 probably about 40-ish.

 

The pico despite being roughly 10" away from the 55 remains between 79-82 during the day where as the 55 has gotten upwards of 88... (not good, I now only run the lights at night and the powerheads are on a much lower setting during actual daylight when temps peak.)

 

Now interms of evap the 55 belive it or not with and open canopy and no sump and no fans during the day is loosing roughly 1.5 inches of water or 4 GALLONS of water a day. The pico on the other hand loses about a quater of an inch or 0.08 gallons.

 

So figure the 55 is losing about 1% water a day where the pico only looses about .04% of water a day.

 

So back the original question will a weekly water change of say 2 quarts or so be good enough to maintain desired pH and Calc in a very low demand system (3 hermits, 3 snails, some softies)?

 

 

Also thanks fab that was an AWSOME READ! Thanks for the input

 

[fab]

You are welcome for the "read."

 

Also, please note the condition I cited in the first sentence...

 

given similar relative humidity, lighting and heat intensity at the water surface.

 

It appears in your situation, these conditions are not met, so the equality of evaporation rate ( inches / day) would not hold, just as your experience shows.

 

Your original question does remain unanswered.

 

You say you are doing water changes to control pH and calcium buffering levels in a tiny ecosystem. How much swing do you think you are observing without water changes? Use that information as the basis for figuring out what quantity of water change you need to keep the quality parameters in check. I can only guess that a quart, which is about 12% change in your 2 gallons of water, is a pretty hefty size change if done on a per week basis. When you get up to 2 quarts you are replacing 25% which is a huge amount.

 

It is unclear whether you gain anything on a net basis by performing such large water exchanges, much less so frequently.

 

Try letting your tank go for a couple or a few weeks so you can monitor the parameters frequently, daily, that you are trying to control with the water changes. Then figure out what you need to do. Of course, if the parameters start getting bad during this observation period, then stop monitoring and fix them.

 

BTW, It will be interesting to see the results of a monitoring period like I mentioned above. If you do this activity, please share the parameter traces and your conclusions.

 

good luck,

 

fab

Edited July 2, 2007 by fab