Evaporation Rate Planning Question

MrMudkip

New member
Basically I want to avoid having salt creep and the need to top off water as much as possible. The last tank I set up 7 years ago had major issues with evaporation and salt creep (maybe because I had lots of microbubbles and a relatively hot light fixture).For the new tank I am planning on setting up I will have cooler lights with LEDs. I don't have the tank yet and am having a hard time deciding between a 38gal or 40 breeder tank (maybe 30 gallon tank too).

Do you think it would be worth getting a tank with a smaller water surface area (like a 38gal 36x12x20) instead of a 40 breeder (36x18x18)? This is a surface area reduction from 432in2 to 648in2. Mathematically, this suggests that I would need to replace water evaporation 1/3 less often with the 38 gallon tank (yes I am aware that I am overthinking this, probably trivial problem but ocd and an engineering degree don't mix too well here). However, 38 gallon tanks also are a bit taller and I have heard they are more prone to bowing out since they don't have that plastic brace thing, which would be more of an issue than a top off. Does this amount of surface area reduction really matter in terms of evaporation or is it more due to heat sources? I figured you guys may have experience with many sized tanks.

gh=ΘA(xs−x)

gh = amount of evaporated water per hour (kg/h)

Θ=(25+19v) = evaporation coefficient (kg/(m2 h))

v = velocity of air above the water surface (m/s)

A = water surface area (m2)

xs = humidity ratio in saturated air at the same temperature as the water surface (kg/kg) (kg H2O in kg dry air)

x = humidity ratio in the air (kg/kg) (kg H2O in kg dry air)

Thank you - Noah
 

RobbyG

New member
Surface area and the houses humidity and Temp play a big factor and half the surface area in the same location will have a significant impact on evaporation. The problem is that you also have to balance out the oxygenation levels of the water. Less surface area will mean less oxygenation. A good protein skimmer will add a lot more oxygen than the surface turbulence, so if you have a skimmer it might not be an issue, so long as the skimmer is able to draw outside air that is low in Co2 and your turn over rate is high.
 

LQT

Active member
Unless I missed it, may I ask why you aren’t considering an ATO (automatic top off) unit to make dealing with evaporation much easier? Using my ATO unit, I only have to refill my 10 gallon ATO reservoir once every 2-1/2 weeks.

Also, I caution you on considering a tank with a 12” fore to aft depth as your aquascaping options get very limited with such little room, front to back.
 

MrMudkip

New member
Would an auto top off prevent salt build up though? I didn't think it would. The only way I though to slow that process is to slow evaporation all together.
Also yeah i am not thrilled about the 30/38 gallon 12" width aquascaping, but I still may go that route if I can get my tank to the low maintenance levels I want
 

LQT

Active member
Salt creep occurs where you have a lot of splashing. Saltwater drying on surfaces due to splashing leaves salt residue/creep everywhere. On your last tank, where did you have salt creep build up? My current tank has no saltcreep build up anywhere as I have minimized splashing.
 

LQT

Active member
Would an auto top off prevent salt build up though? I didn't think it would. The only way I though to slow that process is to slow evaporation all together.

I'm a bit confused here... you are seeking to slow salt creep by slowing evaporation altogether.

Yet an ATO 100% takes care of the effects of evaporation on a tank. Am I missing something here?
 

MrMudkip

New member
well I havent really looked into ato systems. I don't exactly know what they can do or what their limits are
 

LQT

Active member
I place my ATO sensor in the return section of my sump, where evaporation causes the water level to fluctuate. If you’re not running a sump, you can put the water level sensor in your display tank. Based on my level sensor placement, once the low level is triggered, it activates a pump to dispense water into my sump until the high level of the sensor is triggered. The pump now stops dispensing water. The water level in my return section never fluctuates more than 1/4 of an inch.

I use a Spectrapure UPLC-ATO which uses a primary pressure tube and a secondary pressure tube as a failsafe. Another great ATO system is the Tunze 3155 which uses an optical sensor and a backup float switch that serves as a fail safe. I would never run a reef tank without an ATO.

Aside from addressing a minor albeit annoying inconvenience like salt creep, the bigger benefit of an ATO is stability. With consistent automated top off to account for daily evaporation. You are maintaining consistency in your salinity level which is very important if you expect to maintain a successful reef. My ATO probably kicks in 6 to 8 times a day to make up for evaporation occurring throughout the day. If you’re waiting several days to off your tank, think about how much the salinity would swing.
 
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LQT

Active member
I would like to limit the actual rate of evaporation if possible tho

You can get a glass lid for your tank, that would certainly cut down on evaporation. Although that would lead to more problems like less gas exchange, heat retention and blocking of the light as glass lids tend to get dirty very quickly... because of salt creep.
 
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