Might be a stupid question but it has been bugging me

[StirCrazy]

hey I have been reading both sides of the coin on weather pt put your LR on the sand , on the bottom of the tank, or on a rack of some sort. while everyone has there opinions there is no definat reason to why one is better than the other.. after thinking about this something struck me funny about 2 of the methods.

a glass fish tank is suported by the outside edges only that i why you can buy stands that are just frames.. now how much weight can you put in the middle of the tank befor the bottoms fails. to me if the weight is spread eavenly (as in a sand bed) you shoulden't have any problems. now when we are talking about live rock you are putting wight in spicific points. ie, the part of the rock that touches the glass, so if you pile it up you could have a pinpoint weight of 30lbs more or less depending on the shape of the rock. now if we build a rack like some people have said using 1" pcv as the legs we are now suporting the entire rock load on legs.. I saw one that holds 150lbs and only had 6 1" legs so at ~25lbs for each leg I would guess that is a significant spot load on the tank bottom.

now the other method I saw was to place it directly on the sand and when I thought of this.. ya that is probably the safest way to do it as the sand bed will spread the weight of the rock over a much larger area than the foot print of the rock its self. sand will distrbuite a load over a angle of aproximatly 45% so 150lbs of rock that covers a area of 18" X 40" would have a weight distrubation area of aproximatly
26"X 48" which would make a weight of .1 of a lb / square inch or 17lbs / square foot.

Steve

[reefburnaby]

Hi,

I guess...it depends. The weakest area in the tank is the middle of the tank. The tank is supported by the four glass walls...so the middle will have the least support and the most flexing. It is the amount of flexing that causes the tank to fail. Glass is extremely hard (almost as hard as diamond), but it is brittle. Hence, flexing will causing it to crack and break.

So...if your posts are near the glass walls, then you don't have a problem. This is because the torsion arm between the pivot (i.e. the wall) and your load is short.

However, if all of your legs were near the center of the tank...then your bottom plate may have a lot of stress. In this cause, the torsion arm is longer. Incidently, this is why your tank tend to bow outward near the top and center of the tank.

Any who...I have 70 lb of rock and its on a two layer rack. The rocks aren't touching the sand...but some of them are. Its made from 3/4" PVC and there are two racks (one with 6 legs and the other with 7). I kind of like it...I might post some pics but I need a digital camera to do that. SLRs just suck for this application.

Say Darren, how do you like your DC-280 (that is what you have right ?) ? I can't decide if I want to spend $300 for a 280 or $600 for good nikon 950....

- Victor.

[Delphinus]

Just going to throw in some observations to further confuse things.

First, placing rock on the sand, say a DSB, can be a problem if you have any kind of burrowing animal (eg. jawfish). The risk of toppling the entire reef structure is there, plus of course the risk of losing the burrowing animal because something fell on top of him (or "her").

Second, don't forget about different densities. When the tank is filled with water the "weight" of the rock on the glass at any contact point is going to be less than what it would be if the rock was sitting in a tank without water. Add to this that varying densities of rock (ie. some is less dense than others) then you can build up a pretty nice sized reef without really having to lose sleep worrying about whether your bottom plane is going to fail.

FWIW I'm setting up a 75g tank right now and I'm putting rock on the bottom but supporting the taller pieces with smaller rock. About 2" or 3" of sand is planned, and I'm hoping for the best. My existing tank is a 50 with a plenum. I had (until recently, that is) images/smiles/icon_sad.gif a yellow-headed jawfish -- and his constant archeological/architectural excavations finally caused my plenum to fail (not to mention toppling several taller structures). So pretty much everything I say in my "point #1" above I am speaking from experience. Point #2 I am only speaking theoretically from my generally oversimplified view of things.

...

And on another note ... Ok, these snowflakes are very festive and all, but they're giving me a headache. I'm going back to work now!

[ 07 December 2001: Message edited by: delphinus ]

[smokinreefer]

DOH! thank god i dont have a lot of rock in my tank, cuz your scarin the crap outta me images/smiles/icon_eek.gif i keep lookin over at my tank, like its gonna bust open *knock on wood*

[ldzielak]

I setup a 120 in October. I had 2 holes for 1.5" bulkheads(over flows) drilled at one end of the tank, each is a corner and 2 1" bulkhead for the returns at the other, again each in corners. I have no plumbing out sides the tank, so you can walk arround 3 sides. So the point was to have over flows and return line from inside the tank.

I called Seastar and asked what was the best place. The middle of the tank is the weakess place, corners or edges are the best.

They also recomend plywood under you tank with a short pile carpet on top to take up the imprefections between wood and glass. This way you support the entire bottom of the tank. This is the practice they recomend for large bottom reef tanks. For my 120, that is 60"x24" I used two sheets of 5/8" plywood. For larger tanks they recomend 2 sheets of 3/4" or more. Obviously your stand would also have to have cross braces to take the load. With this setup, rock directly on the glass would be no problem.

I have an aragonite DSB. Under the rocks I have almost no sand, on the edges I have 3" or more, so my gobie will not cause destruction of the rocks or kill him self.

[ 07 December 2001: Message edited by: Lee ]