Well different types of commercially made bio-filters should have flow rate, aeration, as well as bio-load specifications for them and I would follow those specifications if you are using a commercial bio-filter.
Flow rate in relation of volume of the bio-filter will dictate the "retention time" but I suspect most of us used to doing media based aquaponics never think of it that way.
I don't expect my answer is of much help to you because I don't know the specifications for building/designing home made bio-filters for a particular fish load and all but you are right that if your bio-filter is too small and/or you flow the water through it too fast, you will not get enough bio-filtration from it. The bigger the bio-filter the more water you will be able to flow through it faster and still get the filtration you need.
It's all about balance. Are you running a home made or a commercial bio-filter? I know there are sites out there that have the designs for making scrubbie type filters and they might be able to tell the flow rates for different volumes of filters or how much filter is needed for different fish loads.
Sorry I don't have an exact answer for you.
Hi TCLynx! I see what you mean. Well, it's only a homemade bio-filter. Anyway I know how to calculate the Retention time but I don't know the ideal period for bacterias converting fish waste. But I agree with you that the bigger the bio-filter the longer the water can stay and the more nutrient will be produced. Could you sent me the links of relevant information answering my question. Thank so much!
Don't worry about retention. Just keep passing the water over the filter. Too fast is better than too slow since oxygen is an important component in nitrification. Also if your pH is low, the toxic ammonia is not usually that big of a problem. Below is a chart that you muiltply your Tan by to find the amount to toxic NH3 you have.
If you are worried about solid build up in the filter, a smaller clarifier will help you out.
I agree that ammonia is not as toxic as the pH drops however, I will warn to not let the pH drop too low because the bio-filter bacteria can completely stall out if the pH is too low and then ammonia will spike quite high and you will run out of nitrate and there isn't much in an aquaponics system that really wants a pH falling down below 5. I generally recommend keeping the pH enough above 6 that you are able to tell that it is definitely above 6. Most people would recommend between 6.5-7 as being good for aquaponics.
FYI the pH range of 7-7.6 does also work for aquaponics and if the bio-filter is well cycled up, well I've rarely seen measurable ammonia in my big system that rarely drops below 7.6 because silly me I used shells as media, only real problem I have with that system has been Iron lock out so I have to add chelated iron to it often.
Well thank matthew for your chart and comments as well as TCLynx's. I agree with both of your ideas concerning retention time and suitable pH for AP. Personally I believe that if the retention time for converting Ammonia is not enough for the 1st cycle of its flow, the water still get converted at the second cycle since water keeps flowing in loop condition.Well, a suitable pH for AP is some where between 6.2 and 7 or 7.5. and the ideal pH for plants to grow well is between 6.2 and 6.6. On the other hand fish grows well in pH between 6 and 9 but the best one 7.