I've had some folks long claim EI is bad for fish and bad for fish fry, bad for soft water fish, on and on........well time for them to eat some humble pie:
Most of the fry gets eaten by the rams, checker boards, cardinals, so I have to stand around and net them as they hatch, if........I'm around. I had poor results rearing fry in ultra pure water and in plain tap that was well aged prior to adding them in a quarantine tank(10 Gal bare bottomed with dual sponge filter, heater etc). I have those hospital net tank inserts, basically a hang on net. Fish fry would get caught/entangled and water changes made it a PITA, had a few squashed fish.
This simple tray float and exchange the water-3-4 times a day seems to work very well with plain old tank water, tank water has NO3 sitting about 30ppm or so, high PO4, about 5-6, and Fe at 1.0ppmm +, Gh about 3, KH 1.5, temp 82F.
I would think and expect higher losses in this tray, where O2 and no filter are less, and where the higher ppm's would be detrimental. But...........
In both cases, I tried with tank water 100% to the new quarantine tank exchange, also plain tap, also DI/AC filtered, I only manged to get 7 fish to adulthood. Fungus and other issues, unexplained death. My question was mostly well, maybe it is the higher NO3 and fry's exposure is much more sensitive?
Could have been, until I started doing this method using the tank water and exchanges. If I'd not had this result, I might be tempted and feel strongly otherwise.
Such unexpected results are often the case. Without some evidence otherwise, we are left to our own devices and assumptions. Now..if someone else came along and challenged me to try it and see, I'd have nothing to lose, I was losing fry anyhow and the fish breed like clock work every 2 weeks.
Since I'm using nothing but tank water, it cannot be due to the N, P, Fe etc.............nor low O2 etc.
I cannot say with any certainly what was killing the fry prior, only what was NOT killing them/leading to mortality etc. We often seek answers, when we should seek the questions. This is 3rd x I've had this same result.
Once the fish get to about 1" or more, they are tough as nails. This takes about 8 weeks.
So this, along with Discus, Apistos, Angels(sorry, not altums), cories, shrimps(CRS/RCS, Amanos) are added to the list of fish I have bred with higher nutrients.
We know we can now answer "are higher nutrients have an adverse effect/s on fish, algae and plants? We can say "no, not in any general way". Thus the risk is minimal.
So for those that wish to claim nutrients are detrimental, where is your evidence? Like the example I stated above, if you have no tried to falsify your own hypothesis, then you'd never know.
Here's my evidence and it's going to be exceeding difficult to falsify the results.