Minimum size for grow out tanks?

My experience with smaller plecos have been mostly BNP's. But since my recent obsession with more exotic species (L128, L129, L200, L239, etc), I've gotten (and lost) some smaller specimens while trying to grow them out in a 15 gallon breeder (24" long - same footprint as a standard 20 gallon). This discussion started in Stan's L128 thread, and it was suggested that this might be a good new topic.

Just for reference, all my tanks are planted. The specs are:

pH 6.4
Temperature 77 F
Substrate Eco-Complete
28W T5 lighting 8 hours a day
Eheim 2213 canister filter
50% wc per week (dosing EI for the plants, along with Flourish Excel)
Feed is twice a day with NLS (supplemented by Hikari frozen bloodworms, fresh veggies, etc, several times a week).

Do you think 15 gallons is too small for a growout tank?

Those are all pretty small specimens, even the 2 Phantoms wouldn't have exceeded 9" fully grown, it does sound a little on the small side but more than enough just to grow them on for a bigger tank i guess.
the bigger worry for me is the losses you've had, certainly not something i'd attribute to the tank size, your pH looks fine, do you know the other parameters, ammonia, nitrite, nitrate?
were they the only inhabitants?
did they all die roughly the same time, a few days after a water change or the night after a feed?
even treated water can contain some nasties that the water company has added for "our" benefit
excuse my ignorance but what is NLS?
how much/how often did you feed bloodworm?
sorry for the bundle of questions but a better picture of the circumstances will help people give an informed analysis.
Hope you get through this soon

ps what about aeration? you mention planted, are you using Co2?

I guess I should have elaborate. I don't measure nitrite or ammonia anymore since the tank is 6 months old (after a move where I had it for a year in the office). Nitrate was 5 ppm last night after I dosed KNO3 (the plant uptake is very high in this tank.

NLS is New Life Spectrum (small fish formula and community fish formula).

The tank has 3 1 inch BNP's, 6 Sterbai Cories (about 1.5 inches each), and 16 tetras (8 black neon and 8 emporers). Guess I should have posted a pic:



I lost an L239 after a heavy feed of Hikari bloodworms (trying to get it to eat). The L128 had been living in it for about 3 months. It died 2 days after a water change and 1 day after a zucchini slice feeding. I feed bloodworms 2x a week. I normally feed very lightly 2x a day.

This is the only tank not running CO2. It's dosed with Flourish Excel instead. I have no aeration other than the spray bar on the left pointing upwards. I don't run aeration in any of my tanks because I am used to CO2 injection. This tank used to run a HOB filter, but it's in my bedroom and I found it too loud and switched to a canister (the Eheim 2213).

Oh, and that pics is about 2 months old. The foreground is all grown in with E. Tenellus now (dwarf chainsword).

If it's an aeration problem, I'm surprised I've not seen problems with my tetras. Oh and I have 8 Amano shrimp in there for hair algae control, and I've found them to be extremely sensitive to water parameters (I basically use them as canaries).