Im working on a 20gal tank and thinking of making a 5.5 gallon sump out of an old tank I'm wondering if because I'll have allot more water mass from such a large sump for a 20gal if I could further increase my bio load?
It's a matter of scale. A sump, if set up properly, will provide for loads of beneficial bacteria, and will expand the water in the system by what's in the sump. But you're talking about adding a 5 gallon to a 20 gallon. So, best case, you can populate the 20 as if it was a 25, assuming you aren't adding territorial species. Sure, put five more neon tetras in the 20. Is that worth what it's going to cost to get the sump set up?
I would take about half the money it would have cost to set up the sump and buy a good quality HOB that's rated for 50 gallons. It should hold more than enough media to deal with whatever you can reasonably put in a 20.
Now, if you're just tinkering with filtration and want to try your hand at a sump, by all means go for it.
A 5 gallon sump is so small it's practically an oversized HOB. You could consider an overhead sump (one that drains from sump down into to the tank, rather than a typical below tank sump where return water is pumped back up into to main tank). Those tanks may be more "fun" to build as you could add emersed plants to the sump and make it look interesting.
I have a 29 gallon sump on a 55 gallon tank. I personally wouldn't bother with a sump smaller than 29 or maybe 20 gallons. You could put a 29 gallon sump on your 20 gallon tank
Also, were you going to drill the tank? Or were you going to use a HOB style overflow, either DIY or pre-made?
I agree with @JayH and @bigdreams. 2 HOB's or 1 large HOB should be more than adequate to handle an overstocked 20g, as it did in my situation. With the proper media and adequate water changes, that would work fine.
Our first tank was a Top Fin 20g kit from PetSmart. That is what started my MTS.
It ended up being overstocked, but the 2x Aquaclear 30's plus a sponge filter handled the excess bioload just fine. I did 50% water changes every 7-10 days and the parameters were always 0 / 0/ 10-30. One Aquaclear 50 or + a sponge filter should handle most anything you put in there. A Marineland Penguin 350 would as well and can be found for around $27-30 online, but they do not have adjustable flow.
If your'e like me and cant leave well enough alone, as the saying goes, then go for the sump build. Starting off on a smaller scale will be a good experience if you decided to replicate it on a larger scale on a future tank build.
I also like @bigdreams suggestion of an overhead filter with plants. I've seen members post pics of these before and also on YouTube. Those are great looking setups and the plants do an excellent job with natural biological filtration.
There are a lot of members here that run sumps that can give advice on setting one up.
What ever direction you go with, post updates or create a new thread. It will be interesting to see the build.
If you do under the stand sump and aren't drilling, then I recommend what I did, which was use an eshopps overflow box with dual drains, which let you set up a Herbie style drain. The second drain line serves as an emergency drain line if first one gets clogged.
I think the suggestion to use a sponge filter is best option both from a cost perspective and from a time is money perspective.