Saltwater Tank Turned Freshwater, Questions

[dantech]

Hey guys,
 
Long time reader, first time poster. I have a pretty unique situation, I think.
 
I bought a used Fluval M90, which is a saltwater tank with compartments in the back, used to hide the heater, the skimmer, etc...
I started curing the rocks for saltwater, and as I'm waiting, i read up a lot on saltwater and end up deciding it's not for me. Too much maintenance and too time consuming, for me. Also, a 36g tank is not very forgiving on saltwater.
 
So, I cleaned it best I could and decided to go the Freshwater route.
 
1)If you look at my tank, you'll see that there's a pump pushing water from the back compartments, to the front display area. My question is, will this cause too much current for fish like bettas, guppies, gouramis?
 
2) As you know, in saltwater tanks, there is no need for filters. I have a canister filter that I got for free. The model is a Jebao 404, and it's rated for tanks up to 100-gallons. Will that be too strong for a 36g tank? Will it cause even more water flow? I don't think it'll cause more water flow  because the tubes will be in the back compartments, but I'm no expert.
 
3)Is too much gravel a bad thing? I have about 2 - 2.5 inchs of Gravel in there.
 
4)If I really clean/boil/clorox the live rock from the saltwater setup (which have already been out of the water for 4 weeks), is there any way I can use them for decoration in my freshwater tank? I love the look of liverock.
 
5)On the back glass, there's nice pink coralline that formed. Do I need to clean that out? It's probably dead by now, but I love the look.

 

[Byron]

I will only respond to questions I have some experience/knowledge of, which here means fish species and water flow.  And first, the substrate depth.  If you intend live plants, you sometimes need more depth depending upon the plant species.  Otherwise, I would suggest less rather than more is probably better generally.  A 2 to 2.5 inch substrate is not too much, so this will work.  However, what type of gravel is it?  Here we get into both the size of the grains, and the composition.  If this was intended for marine tanks, do not use it.  If it is calcareous, it may work for hard water fish, but may not be suitable for soft water.  This partially depends upon the parameters of your source water (tap water presumably).  The GH is very important for fish, and the pH is related.  This data you shouold be able to ascertain from the website of your municipal water authority.  I can go into the substrate more when we have this data, and also an indication of the species of fish you are considering.  These have sometimes specific requirements when it comes to water, especially the hardness (GH).  As for the grain size, this matters if substrate-level fish are planned, like any cichlids, corys, catfish.  It can affect plants too.  I tend to prefer inert sand; this causes no issues for any fish, plants do well in it, and it is very authentic in appearance provided you avoid white (this can upset fish too).
 
To the question about filters and fish, yes, this is a very important aspect of freshwater that many do not consider.  Fish that are sedate, like the mentioned Betta and gourami, do not appreciate currents; they occur in marshes, stagnant ponds and similar, and their physiology is not designed to function best if the fish is constantly bucking a current, as this literally burns them out of energy.  The guppies are less fussy.  As for the mentioned filters, I am not familiar with either, but generally you are fine with a filter (like a canister) rated for roughly the tank volume or a bit more.  If there is a control on current this usually solves any issues of water flow.