Where you get the quotation of the RAS system? How big it will be for your 5000 litre water tank? For perch, as far as I read shows 40-50kg per cubic metre. With the perch can grows up to 800g (7 months), you can raise up to 300 fish at one time.

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Bigger problem will be your ras system. In general 5 fish per m3 is a very acceptabl. Rate

That's why I said your ras system is bigger problem. If you want to increase density you better make sure your system can handle the waste.

If your ras system is not industrial don't bother else you'll have a lot of dead fish or very slow growth.

Most people don't even know what is needed to "clean" the water. Just saying. I'm not implying that about you. But if you were an expert on ras systems you would know how much waste your system can handle and hence how much fish you could put

I am not an expert in ras. But I have been fish farming. I had to learn a bit about ras when I wanted to set up a transit pond last time.

Long story short I end up just changing water whenever my waste levels get too high.

In my pond I stock about 10000 tilapia. At the end of the harvest I can be feeding up to 140kg of food a day. And that is just for 1 pond. Find me a ras system to handle that kind of load.

On a smaller scale you would still have to be able to handle a bit more than just a few kg of food a day. You need to add a bit more because the fish breathe out waste as well and that will happen regardless if you feed them or not.

I have tried a roughly 2000 litre for 100 tilapia n 100 catfish fry just for fun. Made a mistake mixed them in the tank. Tilapia grow very fast n not many catfish survived. By the time my tilapia grow to 600grams, the waste produced is heavy even for 100 tilapia fish.  So 500 fishes in 5000 litre tank growing, that is crazy unless u got high tech waste remover machine. Maybe u try split it up into 2 group once it get bigger. I love the feeding time. Below YouTube show can but nobody giving the trick to do it unless got $$

http://youtu.be/EGlIS-8QBgo

The farm is earthen pond. But we have non stop water flowing in from a river source. So we never have to worry about waste. Because of the flowing water, our ammonia levels never ever reach critical levels. If they start to rise we simply open the pipe to let in more water for a few hours. In effect we're doing a big scale water change.

For aeration, we use a 2hp air blower with aerotube. It is good enough because we have constant water flow. In fact you can use this method as well instead of oxygen injection for your ras. It's definitely the cheaper solution. Pond is roughly 1 acre in size.

Edit: you seem to know quite a bit about RAS, I'm still interested to learn about the ras system. Maybe we can meet up one day and share knowledge? Where are you based at?