As far as I can tell from being an aquarium enthusiast and a hydroponics enthusiast there is not a set in stone rule when it comes to this. The problem is that 2 different fish can produce different amounts of waste (goldfish are notorious for producing a lot of waste), and two different types of plants can have varying absorption rates.
However there are a few thing to go off of. Using testing kits (for Ammonia, nitrate, and nitrite) you can keep a chart of your levels every week and determine if you need more fish to make more waste, or less fish if you have too much waste. From what I could find in a stable aquaponics setup: ammonia levels should range from 0.25 to 2.0 ppm; nitrite levels should range from 0.25 to 1 ppm, and nitrate levels should range from 2 to 150 ppm. Aquaponics is considered a "newer science" so it may take a lot of personal balancing to get everything within the right levels as there isn't a "standard" on how to do it.
Unrelated side notes: Forgive me if I'm being a little nosy but a few things I've gathered from your picture:
Do you have a cover lid for your water reservoir? From my experience with hydroponics, you want to limit light as much as possible in your water. Algae grows wonderfully (and seemingly out of nowhere sometimes) under grow light and will deplete all the oxygen in your fish reservoir which is bad for your fish and your plants.
How close is your grow light to your plants? It may of been that you just moved it to take the picture, but you want your grow light to be as close as possible to your plants.
Do you have an aquarium or wet/dry filter handy? A simple filter like this provides additional aeration for your fish and your plants and also an area (specifically the mesh material that does the filtering) where ammonia converting bacteria can live and thrive. You need these bacteria to convert ammonia into nitrates and nitrites (which are what your plants need to grow).
Are you supplimenting with additional nutrients? Nitrate and Nitrite are great for boosting plant growth, but that is not all they need. Trace amounts of phosphorus, magnesium, iron and other nutrients are needed for plants to grow properly. You may be lucky and find that some of these actually occur in the fish food you are feeding your goldfish, and any uneaten food will get dissolved into the water supply.
I apologize ahead of time if any of this is overbearing, just want to be thorough. Please ignore any of these if they aren't applicable.