Hi and welcome to the forum
Are both aquariums the same size?
Fish grow faster in larger volumes of water (bigger tanks).
Female bristlenose don't grow as quickly or as big as males. You might have a male in one tank and a female in the other tank. The smaller one could have intestinal worms or some other health issue slowing its growth.
The red finned shark will be competing with the bristlenose for food. Put food in at both ends of the tank so the shark can go to one side, and the bristlenose can go to the other side.
Make sure you have driftwood in both tanks because bristlenose and rainbow/ red finned sharks need the wood to help their digestion.
Put a plastic storage container outside in the sun and fill it with dechlorinated water. Add 1 level tablespoon of garden fertiliser per 20 litres (5 gallons) of water. Put some smooth rocks or plastic ornaments in the container of water and leave them there until they are covered in algae. Then move one algae covered rock into each tank and let the bristlenose and shark can eat the algae. When the rock is reasonably clean, rinse it off and swap it for one in the container.
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How often do you do water changes?
Do you gravel clean the substrate when you do a water change?
Do you dechlorinate new water before adding it to the tank?
If you want fish to grow quickly, you should do lots of big water changes. I recommend doing a 75% water change and gravel cleaning the substrate every week. If you have suckermouth catfish in the tank, then you can do it twice a week. In my rearing tanks I used to do a 75-80% water change and gravel clean the substrate every day and the fish grew rapidly.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
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Make sure you clean the filters regularly. Filters should be cleaned at least once a month, and every 2 weeks is better if you keep suckermouth catfish (bristlenose, plecos, etc). However, if the filter is less than 6 weeks old, do not clean it. Wash filter materials/ media in a bucket of tank water.
If you have a filter that has pads/ cartridges in, and the manufacturer recommends changing these pads regularly, get some sponges from another brand of filter and put the sponges in the filter with the pads. Keep them there for 6 weeks and then remove the cartridges and don't bother replacing them. Just use sponges and they will last for years and save you a lot of money on replacement cartridges.
Sponges should be cleaned in a bucket of tank water and re-used.
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What is the temperature of the aquarium water?
Fish grow faster in warmer water and 26-28C (75-82F) is a good temperature for rearing most tropical fish. If the water temperature is less than 24C, then raise it up a bit. Lower temps (24-26C) are better for most adult fish unless they are breeding, then raise it up during breeding.
