Reel McKoi wrote:

Even if his filter keeps the ammonia at 0 doesn't mean there aren't OTHER
pollutants building up in the water. 70 fish in that much water is quite
a load. He didn't mention how often he's doing partial water changes, or
how much he changes at a time.

Agreed. If the bio-filter is working well enough, all that means is you
have no ammonia and no nitrite, but nitrate (particularly at this time of
year when it's not being fully utilized by plants) may be building up.
Nitrate is usually not a big issue, and I still think it's mostly a buffer
problem, but when nitrates get very high it's possible to get all sorts of
strange illnesses.
--
derek