I have noticed that my 2nd stage trickle filter starts to foam after spawning which is quite normal. The foam usually goes away in a day. Since my tank water harness is low at 50ppm, I added 4 calcium carbonate tablet (made from oyster shell for human consumption) to increase the hardness and at the same time buffer the water a little. After adding the tablet, the foaming worsen with foam thickness about 2" above the water level (in the trickle filter). The foam is not very fine and soapy like. I have earlier added sodium bicarbonate to buffer the water due to PH crash.

Could the foaming caused by excessive calcium carbonate dissolved in the water or caused by the milt from the male goldfish during spawning? Other than the foam in the filter the water is very clear. Water parameters is as follows:

Ammonia - 0ppm
Nitrite - 0ppm
Nitrate - 35ppm
PH - 7.0 (slowly dropping from 7.2 4 days ago)
GH - ~80ppm

It's from the spawning.

Consider adding some crushed coral to your trickle filter. that should keep pH around 7.5.

Thanks Betty. How long do you think the foam will go away? Should I remove it manually or leave it alone? I will add some crused corals today as I obtained some yesterday and it is still soaking in some fresh water.

Cool.
I really don't know on the foam. A partial water change would probably help. Fresh carbon may help (not sure on that). a protein skimmer would help.

I just did a partial water change and added a small bag of crushed shell and coral. Also added 1tsp of sodium bicarbonate to buffer the water as our water had zero KH!. Tested the PH to be 7.2 after water change and GH is around 100ppm after adding some calcium carbonate tablet yesterday. I think the PH and hardness will be stable after the addition of the crushed corals.

You are right that the foaming came from spawing. The partial water change did help to reduce the foam in the filter

Water is sparkling clear after water change. The water used to be a little cloudly after water change and takes more than 2 days to settle down. I think it could be due to low PH and KH causing the problem.

Thanks for your help.

So did you put it in the trickle filter?
More shells/coral would be a good thing, cuz they don't dissolve till pH falls below 7.5... so having lots to dissolve is a good thing especially since your water has no buffering capacity. Depending on the size of partial water changes, you may still need to buffer the change water with baking soda to bring KH up to 4 or 5dh.

Yes, the crushed corals are in the 1st stage filter where water flow is the greatest. The bag of corals is just above the suction pipe. I have added 2 tsp of BS to buffer the water as the PH drops slightly after water change. Still monitoring the PH to see whether it will stabilize at ~7.2