Never start an adventure like that with the plan of cutting the competitions throat. You'll end up cutting your own instead.
If you can cut his prices by 50%, then I'm betting you've missed something in the figures.
Here's a list of overhead costs for the LFS I help at:
Rent-$1500.00/ month
Electricity- $2000.00/ month yes that is two thousand.
Water is covered in the rent/ lease
medications- we spend about $500 a month
Foods- $500 a month
Shipping generally cost us $600 weekly. That is for a $3500 order which is about average for us.
Then you have your water conditioners that you'll go through weekly.
Payroll. You can't take care of a store by yourself and keep up with everything else.
You'll have to account for the livestock that dies within hours of receiving them. Wholesalers don't warranty those. They will credit your account for whats DOA.
You'll have fish come in with Ick, ulcers, flukes, and a host of other ailments.
You'll have corals and anemones come in bleached. Corals will have nudis, acro eating redbugs, rock covered in aiptasia and algae, flatworms, uncid worms, you name it, you'll get it.
I'm not trying to talk you out of opening a shop. Just don't open one and expect to cut the other guys throat and still keep your own doors open.