OK, I will help with the filter question first, and then offer some advice on serious pending issues.
The waste produced by the fish, the stuff you are siphoning out of the substrate, has the same effect on water quality with or without a filter. This is where many are misunderstanding what "filtration" does or means. The biological filtration occurs in the filter and also in the substrate, and this involves the processing of ammonia into nitrite into nitrate, which we then remove by water changes. But the solid crud remains, only getting pulled out with water changes that include a cleaning of the substrate. A sponge filter or airstone is not going to improve this at all. The best thing you can do is more water changes. Given the circumstances here, I would advise a W/C of 60-70% of the tank volume once each week.
Removing some of the fish now is another necessity, and here we come into issues unrelated to the original question.
The Bala Shark, species is
Balantiocheilos melanopterus, is a shoaling fish which means it must have a group:
Peaceful for its size, but not a normal community fish. Other fish must be large enough not to be eaten, and able to tolerate this fish’s very active swimming. This is a shoaling species with a pronounced social structure within the group, and must be maintained in groups of at least five fish. Fewer will result in aggression to the point of death of subordinate fish, and/or aggression toward other species in the tank.
I appreciate you may get a larger tank--but you my not. And in any case, the two fish alone now is causing them trouble. You cannot see this, until it is too late. I am sure you care about the long-term health of your fish, so you should rehome these ASAP.
Angelfish and gourami should never be housed together. To make matters worse, the blue and gold gourami are actually varieties of the same natural species,
Trichopodus trichopterus, and this is one of if not the most aggressive and territorial of the small and medium-sized gourami species. I can promise you, this is not going to turn out well, unless you luck out and have all females, but even so, we have had members who had a lone female kill every other fish in the tank after a few months.
I cannot tell you what to do, but I and others can point out problems so you can decide better. I'd be happy to explain any aspect of this, just ask. I know you may think things are fine now...but believe me, this is not at all likely to last. If one of the angelfish is a male, or one of the gourami is a male, they will likely pair up with a female if present, and then...look out.
