I'll reply here because it might still be of common interest.
I understand your reflection about soil/water related causes with the infected seedlings. But miner flies even prefer "tender" seedlings, - and if they detect them they lay eggs on them at a very early stage. It happened more than once to me too.
I am quite aware of the situation you have to face in Cambodia. I know of people who used to work with some projects over there who couldn't believe how developed and advanced Thailand is compared to Cambodia, when they came over - Cambodia is real third world in many senses of the term right?
PH is indeed a matter of concern, but from a purely pragmatical point of view, remember that you were growing successfully. That indicates that the PH can't be that bad - although it is prone to change with seasons (monsoon- versus dry season). But I can probably help you out with this matter on a private/donation way. Until then, if you get to test it - around (close to) 6 is all you need. As for the ammonia content, I believe that plants can tolerate higher levels as the fish. To solve eventual high ammonia levels, best is to promote bacterial activity that will break it down to Nitrates and eventually to absorbable Nitrogen. A secondary pond with "nitrate-hungry" water plants may also be an option. But then again your hydroponical plants can make good use of that nitrogen and hence it mainly and actually turns around promoting bacterial activity. I'd recommend to investigate in aquaponics as well, - but here again there is a learning curve and time to take.
Yeah, as you said... all this can be stretched out indefinately
PS: earlier I didn't mean to "discourage" anyone from any input, I was only saying that there is no way, I'll engage in any kind of futile debates like in the "past"...
