I doubt it will hurt the fish at all, so long as you are properly stocked for a 33 gallon tank, the low volume point.
I am on the smaller side with a 20 Gal tank and a 10 gal bed and a 5 gal bed. Those are filled with hydrocorn and hydroton. That current takes 1/3 of the tank water to fill both. I only have 4 fish in the tank and they are growing but remember to have good aeration the changes in the depth of the water should not effect them, unless they are breeding. The 10 gallon bed holds 45lbs of hydrocorn and the 5 gallon bed (tube) holds only 10 lbs of hydroton. I use a 475 gph statuary pump that will empty the tank every 30 minutes so every 15 minutes it drops to the 1/2 to 1/3 level.
Remember the O2 and water temp are more important than the depth to the fish till it is time to breed.
I would say that you are leaving yourself a small margin for error with all the evaporation your system will experience. Expect to also lose + or - .75 inches a day in evaporation. You will have to keep a close eye on it to make you have enough water in your system for your pump and plumbing to work correctly.
When your tank fluctuates in height the head pressure also fluctuates and you will find that when a lot of water evaporates that your pump is not pumping as much volume as it did before. That change in volume can effect your siphon operation such that it either never starts (too little pressure) or never stops (too much pressure). If your grow bed volume is bigger than your fish tank you may want to consider a sump.
A sump will give you a much greater margin for evaporation and lower the variance of head pressure.
Bill, Affinan is a good guy lots of good technical information but I have had a hell of a time making it to practical use. I have used it as a base then modified it all to make it work. As for the pump check the labels on size and what their lift is. Mine is 475gph with a 7ft lift leaves me about 10gph, mind you it is a statuary pump, but at the 5 foot lift I have now it is pumping about 30gph. I also run the pump constant, no timer just the siphon. You can always go larger and do what I do and add a ball valve at the head to control the flow.
I know most are using other pumps but I found the $45.00 I paid for the Tetra 475 (not made anymore but Home Depot may have it on the shelf) serves it purpose well. I would recommend nothing less than a 300gph but each is different so you need to check the lift on the pump specs also check the outlet size and match best to your water line, which I have at 1/2 and the pump is set for 3/4 so I get a little extra pressure due to that..
I use an EcoPlus 396 (which is 25 watts) which should do fine for you. The 633 (35 watts) will give you a little more gph which you can use make to return some of the flow to the fish tank for aeration. It is an inexpensive pump so you should buy two to have one for a back up. The minimum you want to shoot for is pumping your fish tank volume 1x per hour, so that at 55 gallons the EcoPlus 396 will get that done with room to spare. It is true that if you are pumping 3-5 feet up the volume of water pumped is a fraction of 396 gallons per hour, but it should leave you with plenty for a return to your tank for additional aeration.
Don't forget to get a GFCI outlet to protect you and your fish!
Bill Kelleher said:
I've decided to use a sump. The grow bed will be on top, then the FT, and then a SLO that will flow into the sump. From watching Affnan's videos, it seems like he recommends a smaller pump than is traditionally used with a timed ebb and flow system, but he described pumps in wattage, not GPH, so it's hard to tell. My sump tank will be about 25 gallons and the pump will be about 3.25' from the top of the grow bed I'm pumping into. Any ideas on pump size for this?