Dhu.
It will be very hard to find the answers you are looking for in this forum, and you can eventually end up frustrated by reading all the stupid lies that a lot of guys write here.
7 meters is not shallow, 7 meters is way deep my friend, and it's an ideal depth to do spearfishing, spearfishing is commonly practiced in the range of 5 to 17 meters, if somebody tells you that they do spearfishing at 30 meters you are listening either pipin, carlos coste, or a real dumb teeneager snorkeler writing lies on the internet.
Hunting is a very demanding activity, going down 25 to 35 meters, fight with a 20 kilos fish and going back to the surface is a task for aquaman or pipin and not for exelent free divers as you.
The 15 meters are not far from you, considering that you hunt at 7 meters, I assume that it takes you 10 sec to get to the bottom, 30 secs to hunt in the reef and 10 secs in the way out, well just cut the 30 secs in the bottom, and keep on kicking hard deeper for 10 seconds, after that, kick hard to the surface and you will get your 15 meters, it will take you 20 secs the way down and 20 secs the way up.
But is kind of buring isn't it?, after all, you are a hunter.
The solution is not in this forum among the 40 meters deep hunters and the statics of 6 minutes in the shower just this morning, the solution is in a real and professional course from real free divers, from a real assosiation as AIDA.
The guys who tells you in this forum that going to 20 meters is a shallow inmersion, tells you that cause for them 1 or 100 meters is the same, a fantasy in their heads, what your are doing is a big deal, and you need to be propered trained to safely go for your personal goals. This is not a game or a video play as some who write here makes it seem, this is an activity who claims many lives each year, and shit heads like the ones we regularly read here, encourage real divers to risk their lives.
Do not go for your personal records by yourself, and do not go with the internet's shit head advices, go to an AIDA instructor, or seek for your representation of AIDA in your country, they will guide you safely to reach your goals.
Just think that a 3 star AIDA diver goes down to 24 meters, and a 4 star AIDA free divers goes down to 32 meters, considering that to own the maximum 4 star ranking in AIDA you have to go down 32 meters, grab a card and go rocket speed up, do you really think that this guys go down there to spearefish a barracuda of 15 kilos at 40 meters, fight her, and ascend with an extra 30 pounds of ballast to the surface?????????
Do not listen the shit heads, go to school, your life depends on it.
A guy who spearfish at 7 meters is not a beginer as you think, you are way up to most of the guys here, they go deep only in their bags of potato chips next to their keyboards.
Saludos
Okane