I think I now know what might have caused the recent water quality issue.
There were three things going on. First, there was some kind of an air block in the pipe that slowed down return water flow from the growbed to the tank. I noticed a slow down before we left for the long weekend, and I reduced waterflow from the pump and thought it solved the problem, but it did not. Water level stayed up for a couple of days and mint plant spread across the bed got rotten and spoiled the water.
Secondly, there was a power outage for at least eight to ten hours (realised this after seeing the state of affair in our kitchen freezer :-|). This stopped water circulation (water pump) and the aeration (air pump) while water quality was already pathetic.
Thirdly, since we were out, the maid who helps us with the household chores fed the fish during these days. Fish don’t eat much when they are stressed and you know what uneaten food does to the water quality!
In short, ammonia level increased, dissolved oxygen decreased, water became really muddy making it hard for fish to breathe.
We had one pearlspot (Karimeen) in the tank which is less hardy than the Tilapia (there are 35 tilapi) and was almost dead. I moved it to another smaller aquaponic system I have. Then reduced water level in the growbed to the bottom and restarted water circulation in full swill swing. After a day water looked better and I thought we are good. But fish stopped eating the next day and I found a big dead Tilapia at the bottom of the tank which was dead for at least two days. I did an ammonia test and it was close to 0.5 mg/l, not too bad but may cause damage to the gills of the fish.
On the plants side, this killed a couple of tomato plants, a couple of green pepper plants, and most of the mint leaves. Long beans and ridge gourd lost some leaves and flowers, but survived.
Finally we are back to normal, with plenty of learnings and a bit of disappointment of losing a fish that we had since we started the system.
Some rework in the system is in order.