what was the float with the brass lever for?
Probably just a top up valve.
When you say new, How long have you had fish in the system, was it cycled up before you got fish?
When you say levels are all good, can you tell us what the levels are?
what pH of 6.5 and ammonia 0 nitrite 0 and nitrate ?
How many fish do you have?
How big a fish tank?
How much grow bed?
What kind of grow bed?
Any other filtration?
Any additional aeration?
How much are you pumping?
So how long have you had the new fish? And what are you feeding them?
How many have been dieing? And do they all look kinda like that?
I'd say your nitrite level is high and it could be due to insufficient filtration for the amount of fish. 30-40 gallons of media bed is probably only sufficient for 4-8 fish if you intend to grow them out to 1 lb each. Now you may get away with m ore fish than that when they are small but as they grow you will wind up over stocked. I would recommend adding more grow beds till you have more like 150 gallons of grow bed to go with your 150 gallons of fish tank.
With nitrite up over 1 ppm you should either salt your system to about 1 ppt of salt or do water changes till the nitrite level is down. Nitrite poisoning causes brown blood disease, look and see if gills are brown instead of pink/red and that may help you figure out if it is the nitrite or something else.
As long as your source water doesn't have lots of salts in it and you haven't been dosing heavily with seaweed or salted in the past. 1 ppt of salt will do no harm to anything except perhaps strawberries. Most plants tolerate a salt level up to about 2-3 ppt just fine but if you are going to use salt more than once, you will need to get some way to measure it's concentration.
If your tap water is on the salty side or if you have salted or used lots of seaweed you will want to test your water for salt levels before salting.
You mention that the problem system is outdoors, Did the water get cold at any time? Tilapia don't thrive if the water is below 70 F and below 55 F you start to get into the danger zone since their immune systems can be compromised if the water dips below 53 F and I've had Blue Tilapia (supposedly the most cold tolerant ones) just die when the water got down to 48 F. If that system spent a long time at temperatures below 70, that might explain why the fingerling in the picture looks a bit malnourished, tilapia don't eat much when the water is below 70 and if they were little guys they wouldn't have had much in the way of energy reserves to keep them going if the water was cool for too long for them.
Dukie, how long have you been cycling? It can take easily 6 weeks to begin to be stable and up to a year. As the "good" bacteria grow and take charge these things tend to control themselves. Time is on your side. Now that my system is 9 months old I get hardly any fish deaths (maybe 1 per month and generally a very slow grower indicating some other reason). That was far from the case 6 mos. ago. You may just be pushing things a bit. I went thru 20 comets, 30 bluegills and about 40 catfish before things settled down. Now testing the water is boring as it always seems on the money. Hope this is relevant.
BTW, mine (dead fish) get ground up along with all the other waste and get composted. Fish pieces parts are great for the compost but should be ground and mixed well with the greens etc. Waste not, want not.
Dukie, fish with Ick have what looks like white pimples. Ick is a parasite, not a bacteria.
Keep in mind if you use anything in your system to kill bacteria, you will kill your system and have to start cycle up all over again.
To deal with Ick, you can bring your salt level up by 3 ppt and keep it there from 1-6 weeks depending on temperature. Live cycle is shorter in warm water. This will likely kill salt sensitive plants but if the fish you have are tolerant to salt (which tilapia are) the salt may actually help them. Make sure it is NOT Iodized salt.
Here is a blog post about salting for fish health.