Fan placement and results?


Building my canopy over my 45g high. (2) 96w PC's and (1) 175w mh mounted in the canopy with reflector with remote ballasts next to the stand. The lights will be 8" of the water, and tank has center brace. PC's will be on for 10hr and MH for 5 or 6hr if thats enough time for the mh. I run the tank temp at 80 and in the summer its at 82-84. I can't have a chiller and ac is on only in the evening. The canopy is wood with a open back that will have egg crate to keep the fish in. How many fans should I have, which direction should they be and what type of vents should I have?
1) Have 1 or 2 4" fans blowing in and have the other end vent openings.
2) Have 1 or 2 4" fans blowing out and have the other end vent openings.
3) Have vent openings on both ends and have fans blowing out the open back.
4) Have vent openings on both ends and have fans blowing in from the open back.
With regards to the vent openings, should I drill out small holes in a 4" circular patern or cut a 4" hole and put a speaker fabric to collect lint and keep the light from emitting?
Pics would be helpful and any other alterations.


Active Member
Well - sort of opening up a can or worms with the fan discussion - but that's okay - can of worms is okay.
All I will say about keeping an enclosed area cool - when there is a huge heat source within - is this.
Heat naturally rises.
Our aim is not to cool the bulbs/lamps - but to pull the hot air UP and AWAY from the water surface. Period.
I'm a firm believer of mounting fans in the ceiling/roof of the canopy ( again heat rises ) and to install the fans so they BLOW AIR OUT of the enclosure.
Doing anything else besides this - will just blow air around INSIDE the hood - and this is not desirable.
You want to get the hot air OUT - not blow it down towards the water surface.
I mounted two IceCap variable speed 4" fans in the roof of my DIY canopy a few months ago.
Both blowing air OUT the top.
Warm air does not hit me in the face when I walk by the tank.
Warm air does leave the enclosed area as cooler air is PULLED in at the very back of the unit - down at WATER SURFACE LEVEL.
This allows the cooler air to natually be pulled up across the water surface - passes by ALL lamps/bulbs - and warm air is blown out the top.
This also creates a "positive" pressure within the canopy - and a positive pressure is what you want. Especially up near the fans where the "work" get's done.
I will debate this until I am blue in the face - because I KNOW IT WORKS and I've taken measurements.
I have over my 75 gallon tank.
Two 250Watt Ushio 10,000K Metal Halide Lamps
Four 110Watt VHO
That's a total of 940 watts of lighting over the tank.
The tank water temperature has not risen more than 2 degrees.
I run (2) VHO from noon to 10:00 pm
I run (1) MH from 2:00 pm to 7:00 pm
I run (1) MH from 4:00 pm to 9:00pm
I run (2) VHO from 8:00pm to 10:00pm
The house ambient temperature is held at 70 F


Broomer.. I didn't read all of your responce but have in the past. My question is did you every try to place the fans in any other configuration? It wouldn't make sense to turn them around and blow down onto the water but what about across the water and not necessarily the bulbs. I understand it work for ya but did you ever compare it to any other setup? Can't remember if you said you have.


Active Member
I think I came off a little too strong on my first reply.
No - I didn't try it any other way than the way I chose to do it.
It made no sense to me to even try it.
Blowing room temperature air on a metal halide does nothing in my opinion.
These lamps are designed to run hot - even if you blow 70 degree F air on them - it's not like they would run much cooler now.
These things generate A LOT of heat. That's the nature of the beast.
My goal was not to reduce the heat that the lamps run at, or even cool the air inside the canopy.
My goal is to get the HEAT OUT.
Up and away from the tankwater.
An exhaust system of any kind is designed to remove whatever it is you want to exhaust - be it air, fumes or heat.
I've yet to see an exhaust set up that does anything but "pull" the air past the area that needs air evacuated - and then discharging this air out the top stack or area of the enclosure.
Cooling an electrical appliance or computer - that's an entirely different animal. It makes sense if you're wanting to increase the life or efficiency of the device - by "cooling" it ~ but that's not my concern here. My main concern is to keep the water from getting warm, and to decrease water evaporation - to some degree.
But as always - there's more than one way to skin a cat ~ which is an odd expresion in itself.
To each his/her own.


I agree with you on all of that but here is another way to look at it. We sweat to keep ourself cool on hot days. When the sweat evaporates in cools the body. Same concept with the tank. If you are unable to pull enough heat from the tank by blowing the air out wouldn't blowing a fan across the tank, not necessarily the bulbs cool the water. I plan to have mine blowing across the water( i think, not sure now) and have a vent above the mh for the excess heat to rise out of the top of the canopy. The cool 72 degree air will go across the water pushing the hot air up and hopefully out of the ac vent on the top of the tank and at the same time it will be cooling the water. I would think that by doing it this way you would be able to cool the water more if you needed to??? Not sure about that. I'd test these different ways to do it but I don't want to butcher the hood i just built and i would have to have different placement depending on how i wanted the air to flow.


Active Member
That's very true Fishfood
Our body does maintain temperature by producing sweat.
As the water moves out to the skin - it contains heat.
For water to evaporate - energy must be added to it.
In order for this water to evaporate - it must "give up" this heat.
The water evaporates because there is energy added to it in the form of heat - the water molecules become excited - or more active - and the polar liquid water molecules are no longer "attracted" to each other as they are in a liquid form - and they become a vapor. When these water molecules are in a vapor form - they exist independently. They're still the same old water molecule, 1 oxygen and 2 hydrogens, but there is no attraction any more. They are a vapor. The amount of energy is still in the vapor form as it was in the liquid water molecule form - it's now called latent heat. You can't see it - it's latent - but it's still there.
As it cools - it once again gives up the latent heat and condenses back to a liquid form of water.
So water evaporates because of temperature.
At a given temperature - on the very molecular level/surface of our tankwater - the number of water molecules that exist in a liquid form is equal to the number of water molcules that exist is a vapor.
These molecules are changing form all the time.
Some are shooting off as vapor while others just near the surface are condensing back to liquid.
It's called equalibrium.
At increased temperatures - more water molecules jump off into the air as vapor - because there is added energy to the liquid water.
At decreased temperatures - fewer water molecules jump off into the air as vapor - due to less energy being added tothe water.
But here's the tricky part. This is assuming there is NO AIR CIRCULATION. If you add air moving into the equation - you have fewer water molecules condesing back at equalibrium.
Why ?
Because we blew them away.
These vapor molecules are no longer hovering just above the water surface. They are being removed from the equation - and end up floating around the room as humidity.
They can not "condense" back into the tankwater - cause they are no longer near the tankwater surface. Their gone, and will end up condensing on your cool coke bottle on the coffee table in the form of water dropplets.
So when we say we are "cooling" the tankwater - by blowing air across the water - we are in escence doing just that. But also we are removing the water from the equation.
We're removing the water ( in a vapor form that contains the latent heat ) and thus the tankwater "gives-up" it's energy.
The tankwater gives up heat.
It does not necessarily get cooler.
It just doesn't get any warmer than it already is.
But in doing so - we lose water.
If we actually wanted to "cool" the tankwater - we would need to actually DROP it's temperature - either with ice or a chiller.
In other words - blowing air across a container of water - doesn't really cool it - it just allows the water to leave in the form of vapor - and steals some of the liquid water's heat while doing so.
The water evaporates - loses some of it's energy in the form of latent heat in the vapor - and does not get any warmer.
Same thing when we put our wet hands under a warm air blower in some public restrooms. You're removing the water. Drying your hands. Not cooling the hands.
Does that make sense ?


Here is what i'm going to do. I'm going to try two 3" fans. One on each end of the tank. I'll also put the ac vent cover on the top above the MH. I'll try them blowing in first, blowing out second and then try it with one in and one out. The tank is only cycling anyway so i have time to mess with it. I'll do the test once i get the fans and lights installed.
BTW Broomer5 - I'm not trying to prove you wrong or me right because i really don't know which is better. This few day test may not tell us anything at all. My house temp fluctuates between 70 and 75 throughout the day anyway so it won't be exact here.


I will debate this until I am blue in the face

Broomer will do this with about everything. (lol)
I do agreee with him on this. fans blowing out.


Not to muddy the water any, but you can cool the water in the tank by blowing air across it. It is the same thing as a "Swamp Cooler" and they work. Boomer, what you said is correct; the act of evaporation does take energy. That energy, in the water vapor, being removed from system is what takes the heat away from the water in the tank.
The big issue I have with using this method is that you loose a LOT OF WATER. I personally run my AC to keep the humidity down in the house. (I live in Houston, TX where the humidity is always high)
In my opinion, if your water is not hot without your lights, then your tank, if properly vented, should not be hot with your lights.
I have a 150 gal tank with 2 400Watt MH and 4 (5') VHO. My tank temp increases only 2 degrees with my lights on. I run them for about 10 hours a day.
Pulling the HOT air out the top IS the most effecient way.