So how many fish can I have in my 300 gallon tank? or how much Raft do I need to support 60 lb of fish? This is the place to share the numbers for designing a well balanced system.
I know the numbers for Flood and drain media so that is what I will share and I'll let the experts on rafts share that info.
Standard rule for media systems is (Actually this is MAX and only for the skilled fish keeper.)
3 kg of fish per 100 liters of flood and drain media (those fish can be living in 50-100 liters of water, and a sump tank could be necessary to provide extra water if the fish tank is on the small side of the numbers) So to convert that to measure that we think in more commonly.
5 lb of fish per 25 gallons of flood and drain media.
A more appropriate recommendation for fish stocking would be 20-25 fish (500 gram grow out) per 500 liters of grow bed or 1 fish per cubic foot of grow bed with a planned grow out of 1 lb. Total grow bed and fish tank should be of equal volume or if a sump tank or indexing valve is used there can be more grow beds than fish tank up to twice as much grow bed as fish tank.
Perhaps this might be a little conservative but it is better to start light on fish while cycling up a new system. Once you get comfortable you can decided how best to keep the fish.
A big note here. Those fish weights are for the planned final grown out weight of fish in a system.
Example, say you have a 300 gallon tank and 300 gallons of flood and drain media filled grow bed.
Say it's a simple system pumping from the fish tank to the grow beds which drain back to the fish tank. Say you are growing tilapia and you plan to grow them out to 1 lb if you can.
I would say stock only about 60 fish. (I'm not into planning on lots of fish deaths so I would only personally pad this number by a few fish-I would not personally stock 100 fish and plan on loosing 40.) Again this is assuming that you will grow all those fish out to eating size in that same tank. Again these numbers are a bit skewed, Kinda figuring people would start eating their tilapia a bit smaller than a pound to make enough room for the other fish to get to a pound. Also, tilapia tend to allow people to pull off higher numbers than are really possible with most other types of fish. In such a 300 gallon 1:1 system I am currently growing only 30 catfish.
They start small in a new system and give you time for the bio-filter to cycle up to the load.
Some other handy numbers for figuring out pump sizing and aeration.
You want your water pump to cycle the equivalent of the volume of you fish tank each hour at the amount of head you are requiring of it. So for that 300 gallon system, if you run the pump continuously and use auto siphons to drain the grow bed, you need a pump that will do about 400 or 500 gallons an hour. If you wish to use a timer and flood all the beds together 15 minutes per hour, you need a pump that will move that 400-500 gallon in15 minutes so you need a pump that can do 1600-2000 gallons per hour. Another option is an aquaponic indexing valve where you could pump for a period of time to each bed in sequence. Figuring out the pump required for this will depend on the indexing valve chosen and the timer one wishes to use.
On pumps, do some extra research, a cheap pump often uses more electricity so it may cost more in the long run than the costly pump. Always look for performance curves to tell how much flow can be expected at different head heights. Remember that small plumbing will reduce efficiency, never restrict your pump output.
On to air pumps. This will definitely be different than the DWC system since for a media system, you only need to aerate fish tanks/sump tanks. Aeration of tanks helps keep solids moving along and not collecting in with the fish. Aeration will also assist the fish in being able to metabolize food and grow faster. If there is not enough oxygen, fish don't eat as well. Highly aerated water also assists the bacteria where ever they may form to convert ammonia and nitrite to nitrate. It also assists in keeping the water circulated to avoid dead spots and brings bottom water up to the surface for better aeration in that way (it isn't just the bubbles contact with water that aerates.)
Aeration numbers. Simple rules of thumb I've been told.
You want 1 cubic foot per minute of air for each 400 gallons of fish tank. You need 1 psi to push air though the air stone and 1 psi to push air down 28 inches under water. So for the average system you want to figure out how much air an air pump will provide at 2 psi and then figure out how many cubic feet per minute you need to provide for your system volume.
I have a 60 watt air pump that provides 2 CFM at 2 psi. That air pump runs all the time. Normally it runs on mains power but I have installed a relay so that if mains power goes out, I have a deep cycle battery that will keep the air running to my system for at least a day. (So there is aeration and backup all in one air pump plus the appropriate batter, charger, inverter and a DTDP relay.)
Air I'm sure is a completely different story in Raft culture and I've heard of raft systems being run almost completely on air with minimal water pumping. But I know almost nothing about blowers so I'll leave that explanation to others.
Some notes about plumbing. Gravity drains always need far larger plumbing than their pumped counterparts. For example. If you are pumping into a tank using 1" plumbing, you had better make sure your gravity drains and overflows out of that tank are far larger than the 1" inlet or the tank will overflow. Don't make your overflows too close to the rim of the tank or they might not have a chance to work before the tank actually overflows over the top. You need enough fall or height for many gravity drains to work, it might not have to be a lot but you can't drain uphill by gravity. So if you are gravity draining from a fish tank, make sure the place to which the water drains to is lower than the high water level in the fish tank or it won't work.
If in doubt about plumbing, go bigger. It is easy to reduce down later if you need to but if the hole or bulkhead is too small, it can be difficult to up-size later.