Backyard Aquaponics
A place to discuss aquaponics

How do I know how much I can plant is there a ratio for how many fish in a tank and how much u can grow on how many fish

Fast growing fish such as Trout & Barramundi require 25L+ of wet gravel per fish to get them safely to a plate size of 500gm. Slower growing fish such as Silver Perch require 20L+ of wet gravel per fish.

If your grow beds contain the widely recommended gravel depth of 300mm with a maximum flooded level of about 250-260mm, this will give you ratio of fish, and therefore nutrients, per sqm of grow bed surface area to successfully grow most commonly grown veggies.

When planting out the GB's in a new system, it's recommended you only plant leafy greens for the first few months. In a well established system I recommend to my customers that they only plant out 1/3 of the GB surface area with fruiting plants, ie: Chillies, Capsicum, Eggplant, etc and the remaining 2/3rds with leafy greens.

My personal suggestion is to avoid growing tomatoes in aquaponics GB's altogether. They are very efficient at sapping more than their fair share of nutrients from the system. Grow them in a wicking bed instead.

+1 - tomatoes fast expanding and probing / pipe finding roots are also a PITA. If not wicking beds then dutch buckets or something where you can better manage them.

Mr Damages comment about leafy greens & herbs is particularly worth noting. It takes time for things to build up in a new system.
Too many posts have "what is my deficiency" and "why no tomatoes/capsicum" etc without considering that things take some time to ramp up and basically a new system is deficient (unless you supplement). Nitrates - which the leafy greens and herbs mainly rely upon - are the first things to be present at sustainable levels.

Both the above responses are good advice.

The ratio you are looking for is actually based on feed input to plant output. The number of fish doesn't help much unless we know how much they are eating each day. I believe that the normal daily max feed quantity is between 1% and 2% of total fish weight. UVI gets a ratio of 60+ grams food per square meter of bed, but that is based on solids removal and some denitrification in filters. Wilson Lennard gets about 17 g/m2 with solids inclusion. Both of these are leafy greens production. fruiting plants (tomato, peppers, etc) dont seem to me to fall too far off the ratio, once the system has developed (can take months or more) if you allow the full spacing of the plant to be counted in the bed. By this I mean that tomatoes would normally grow at approx 4/m2, so even if you plant them more densely and vine them out, you need to allow enough feed for the full spacing. That's how I make sense of it, in any case. I have grown 6 vineing tommies in 1/4 m2 media bed, but I supplemented, and fed, heavily. They were so thirsty for nutes that they would build up roots on the surface under the inlet pipe fighting for first dibs. Especially a new system, I feel that Blood&Bone is your friend, together with some seaweed extract.

This PDF from Wilson Lennard will help you understand why the range is so high, and talks about supplementing.

The amount of fish the system can safely sustain is primarily determined by the amount of wet gravel (bio-filtration) in the system, ie: the 20L+ and 25L+ of wet gravel per fish I mentioned in my first post. Then there is a recommended ratio of wet gravel:water that needs to be taken into account, this will give you the fish:water you are looking for.

If you calculate the total wet gravel volume in your grow beds (bearing in mind you should have a 40-50mm dry layer) then try and achieve a 1:1 ratio of wet gravel to water in the system, you'll end up with a well balanced system.

You can go slightly lower on the wet gravel:water ratio and still have a successful system. Just as an example, a single IBC system with a 300mm GB gravel depth, generally runs about 280-300L of wet gravel to 500L of water, giving a 0.6:1 wet gravel:water ratio. I wouldn't go much lower than that. This ratio gives a good volume of water to each fish, but can leave the system more prone to nutrient deficiency issues, especially if growing plants with high nutrient requirements, tomatoes etc.

On the other end of the scale you can run the wet gravel:water ratio as high as 2:1, but be aware, if you stock with fish to the system's max safe stocking capacity, as determined by the amount of wet gravel, then the fish stocking density in the fish tank will be high, ie: less water per fish. This means less safety margin, Ammonia will build up quicker in the fish tank if water circulation stops, and oxygen will be depleted quicker if the air pump stops.

That is helpful as I have been trying to work out my numbers of trout for my newly cycling system
Based on my system of 2GB's at 1.2m x1.0m plus 1GB x1.2x 0.9m with a of 0.3m total depth with a 40mm dry zone this would give me a wet gravel volume of 905 litres with a full IBC FT of 1000 litres (plus another 150 litres in my RFF not sure if this counts in the calculation?) which is close enough to 1:1 ratio if you don't include the RFF. This would mean that theoretically I could carry 36 trout. Is my maths correct??

Are the 1.2m x 1.0m GB's made from IBC's?... If so, and they have a 300mm gravel depth and 260mm flooded gravel depth, then they will have a wet gravel volume of 270-280L, so can safely sustain a max of 10x Trout each.

I've run Trout off IBC GB's, as described above, and the moment the average size of the fish reached 500gm I started getting constant low level Ammonia readings, which continued to climb every day, requiring the harvesting of a couple of fish about 6 weeks prior to the end of Trout season, followed by a couple more about two weeks before my main harvest when the water temp signalled the end of the season.

Very good thread and some interesting answers thank you!