I'd like to offer a few comments here which I hope will be helpful. First, back to your filter question...for a small tank (100 litres or 25 gallons is "small") you might find an external canister a bit much. You mentioned your present filter is a Fluval U3...if I were filtering your tank I would not go above a U1 or something comparable; I'll explain why momentarily. Internal filters do take up space of course, but on small tanks I have always found them handy. I use a dual sponge filter on my 20g and 29g tanks, and a small internal Eheim
Mini Up on my 33g.
This latter is a recent acquisition, to replace the little internal Eheim sponge that I've had in this tank since 1995 that began falling apart (the connections have just worn with repeated use/cleaning) which is no longer available. I found the Mini Up in a local store, and dec ided to give it a try, even though it is rated for nano tanks (though to me the high flow would seem too much in such a small tank...). This is just a dual sponge insert with a small motor that avoids the need for an air pump. I am very impressed with this filter. Here's a link:
https
/www.eheim.com/en_GB/products/technology/internal-filters/new-mini
Now to the filtration issue itself...in a planted tank you will not need chemical filtration, and biological will be over-kill if it is too great. Mechanical is the only real need, to move the water around. The intended fish should be your guide to how much current you will need. Current is extremely important to fish health, especially in small tanks, as the fish have varying needs and in small tanks may not be able to adequately escape strong flow. You mentioned over-filtering a tank...yes, this can cause issues. For one thing, there is absolutely no benefit to over-filtering any aquarium. Provided the filter is sufficient for the water volume and fish density, it will do its job adequately. Increasing filters has no benefit except in emergencies. The bacteria cannot somehow perform more or faster than the level of bacteria suited for the biological needs. And then there is the water current issue, which can be very detrimental if it is too much for the fish species. And faster flow through a filter is also detrimental to the bacteria in the filter itself.
Now to your fish. I would not recommend gourami with guppies, this does not always work as the gourami can be pestered by the active colourful guppies charging around them. I would also be careful with Dwarf Gourami, which is still a species that may carry the iridovirus unless you obtain them direct from a reliable breeder. The Honey Gourami is a better option, and in your tank you could have a trio. Gourami do not like water movement, as they occur in quiet swamps and ponds naturally, so another bow to the filter.
I second the change to sand substrate, especially for corys. Avoid white, this is unsettling to fish as it is not natural. Play sand works very well, it is smooth, and very inexpensive. The aquarium type sands in darker colours will be considerably more expensive.
I would see if you can pin down your hardness for your source water (check the supplier's website) just so you have the number. But if it is on the soft side, you have a wealth of fish possibilities, but avoid livebearers and any others that need harder water for the mineral content. Most of the small to medium sized characins and rasbora, with a group of corys, are possible.
We can consider plants once we know the lighting. Those mentioned are low-light, but you can probably expand your options a bit.
Byron.
