Ideal have it what Dr. Roy says, however, practically most mantis keepers don't have the right aquariums to accommodate the Lysiosquillina sp. So fill the tank to nearly half as its tall... you may maybe exceed by a several cm from half way. Most cases you will get a small L. maculata and you can control their growth by how many times you feed it. The more you feed the quicker the L. maculata would grow...less you feed the slower.
Again a trick that i do, to have it dig were i want it to start are the ends of the tank. I put live rock on the surface of the places i don't want it to start. Or i put a temporary floor tile. It will dig on the expose sand only, based on what mine have done. So, its a compromise to what Dr. Roy said...just that instead of a vertical burrow it will just go as long as the tank horizontally. Then it is 1.5x as the animal is long...it will dig down vertically to were it would start digging to a J shape. Once it hits your mesh it will dig horizontal till it hits the other side of the tank. Once your L. maculata has started the burrow construction. Having his entire body to fit in his burrow, thats the time i remove the tile or i rearrange the live rock to my desired locations. Once its down there in the substrate...it can do whatever it wants. Eventually would come to the surface having made its second entrance. As it will always have one door close and their beaty little eyes looking at you from the other opening!
Side thought, always wanted to try the Fx5 but seen those up close large bloody canisters! Pretty cool! 
G'day Mantis
thanks for the swift response again =)
Can definitely see what you mean by most practical aquariums not being BEST suited for a L. maculata, considering there potential size and them being burrowers of soft substrate...all part of the challenge!
I will add an extra 10cm maybe 12cm of depth to the Med/Fine coral sand that I might be able to pick up today, only reason I didn't get it all yesterday was because of the cost...25kg's of Coarse coral sand & 75kg's of Med/Fine coral sand cost $265...more then I anticipated but hardly a set back
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It makes sense that, "the more often you feed it the quicker it will grow/ the slower you feed it the slower it will grow" (I can't believe how some people believe animals will only grow to the size of the TANK they live in.....baby great white shark anyone ?!? :lolspin: ).
Your advice seems infinite Mantis
as far as allowing the L. mac to dig in certain place, I noticed in your tanks there seemed to be transparent pipes sticking out of both of there holes, do they go down deep or are they just there to sort of give your L. mac's a place to start, the tile Idea is a good one
something I think I will give ago. I certainly don't want him making the entrances and exits to his tunnel behind or obscured from site by the Live Rock....and I don't really want to disturb anything in the tank once the L. mac is in....
Just about there homes and there burrowing specifically...I know I shouldn't and well I won't if it's bad to do so or against recommendations but anyway, I was THINKING about designing a tunnel for them, running the length of the tank with maybe 3 outlet points and wide enough for plenty of movement within it, its just that if I was capable of doing something like that I could create 2 advantages for myself and the L. mac
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Firstly it would create a rigid safe burrow (not saying that he can't do his own job well enough) but I do worry that if for whatever reason I move something within the tank it may cause one of his burrows to "cave in" and possibly around him, trapping him in the process. Secondly since I would be the one creating the tunnel I could design it for easy viewing a.k.a making the exit/entry holes for it in good viewing spots and even do a 1/2tunnel system along the whole front of the tank where I could see him under the sand and in the tunnel system
?!?!
EDIT: My 2 main worries about creating a tunnel system myself for the L. mac is that a) The L. mac won't use it...and it would be rather large so removing it once he is in would be...difficult at best. b) The cleaning of the inside of the tunnel I make once its inside the tank, if the L. mac does use it would it be something that the L. mac will keep clean inside himself, just expelling out of the tunnel any unwanted bits of waste ?
On that side note of yours....the Fluval Fx5 you stated you have never owned one, but finding there stats online is fairly easy and from knowledge what is on the box is NOT that different to the truth so you can GO off the boxes stats, there is an AWESOME Ehiem Pro vs Fluval Fx5 thread out there, FLuval won
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Anyway... knowing that they are a powerful canister filter and also knowing that this tank will house mostly substrate...am I going over board (in a BAD WAY only though) by getting one this big, so far it seems the tank will hold a lot less water then say what a normal level substrate bottomed tank would hold.
Cheers mate your help so far has been priceless
